Here is my idea. A Dragon of the Moon and a Remnant of a World Part 1: Sieg's arrival in Remnant.
Sieg after his adventures in Fate Apocrypha is allowed to reenter the world by the God of light after the World of Type Moon ended from it's ashes came the world of Vytal remade by the Gods of Light and The Gods of Darkness, and centuries later the God of Light after realizing what happened to Remnant and how the inhabitants of it's world is on the brink of extinction the God of Light sought out one who would be his champion and become a hero for a new age and after the Heroic Spirits in the Throne of Heroes turned his request down the God of Light had found Sieg in the Reverse Side of The World. The God of Light after talking to Sieg asks him to help save his world and when Sieg accepts he is given new powers but also keeps his old ones. But Sieg can only master them over time and after he unlocks them with intense meditation.
A Dragon of the Moon and a Remnant of a World Part 2: Sieg's Powers: He possesses first-class Magic Circuits. Sieg is a being whose purpose was to have Magic Circuits as the foundation of his body, so his ability to comprehend information is great. He is able to go from being less aware than a baby to gaining knowledge at an abnormal speed in a span of days. He is made to take in mana from the atmosphere and provide it to Servants, and he is able to manually cut the link upon gaining intelligence. Instinctively knowing magecraft, he is able to use the spell "Logic pathStraße/opengehen to wish for a result of "destruction" on the glass holding him, causing his magical energy to transform it in a way for the least "destruction" required. Able to understand the mineral the glass is constructed from, it becomes as brittle as a weak piece of wood. When Sieg tries to protect himself from Gordes, he uses this technique to defend himself. His palm became the barrel of a gun - the sheath of a sword - and what it fired was a bullet or perhaps blade that would not only destroy Gordes' arm, but mercilessly devour his heart. it is also said that Sieg can use basic magecraft, such as suggestion and healing magic.
Unlike regular homunculi built to be functional, perfect lifeforms, he possesses many physical defects, and while his circuits are first-class, his body is not able to utilize them without failing. Meant only to supply magical energy and not act as a fighter, he is physically weak and will only live three years at most. After gaining Saber of Black's heart, his appearance physically changes and he gains a stronger constitution which leads Rider of Black to claim he can live for more than 100 years. He has special black Command Seals which appeared on the back of Sieg's left hand known as Dead Count Shape shifter: Dragon Revelation Command Spells which allows him to take the form of the Heroic Spirit Siegfried for 180 seconds (3 minutes). He can only take form using the Command Spell for a limit of three uses. However they can be used as normal Command Seals, but does not use them that way towards Rider.
His parameters, experience, technique, personal skills, Noble Phantasms all are replicated perfectly but he cannot match a true Heroic Spirit(yet). His usage of Balmung does not appear impeded by this. He requires adequate magical energy stored in his body and the use of a Command Spell to take on Saber's form. Using Balmung is enough to deplete his reserves completely, causing him to lose Saber's form even as his magic circuits desperately cling to "remain as Saber." His armor and Noble Phantasm disappear, causing Saber of Black to vanish from the world. The process brings extreme pain to his flesh, bones, and nerves, and he loses any ability to move or retaliate. It is possible to transform again, but his body cannot endure to do so consecutively.
However, after getting revived by Frankenstein's lightning imbued by her will a piece of her is reborn inside him it ceased to hinder him. Later granting him the perpetual motion machine ability of Bridal Chest, he used it, during his battle with Karna and later Shirou, Sieg managed to use Balmung so many times consecutively only because he continued to instantaneously pull out the sword's power while replenishing his mana with "Galvanism". While the Servant version of Siegfried cannot instantly replenish the mana he expends, his sword's accumulation of prana is overwhelmingly faster than that of an average Anti-Army Noble Phantasm, but still not as fast as Sieg. In terms of speed of his Noble Phantasm use, Sieg was closer to that of the real form of Siegfried than even the heroic spirit copy. The living Siegfried, on the other hand, had his inborn disposition combined with the transformation of his heart into a Magic Core due to being bathed in and drinking a dragon's blood, so he could draw forth magical energy and fire the slash wave even faster than Sieg. Unfortunately, the moment he was summoned as the Servant Saber, that special characteristic of his was highly degraded. When Sieg battles Lancer of Red, he is able to use Balmung together with Galvanism to great effect and is able to activate Balmung consecutively. After witnessing two back-to-back attacks on the level of Brahmastra Kundala, Lancer decides that there is only one way to win and uses Vasavi Shakti. Balmung would have been penetrated in only two seconds, but Sieg used the power of his last command spell to dramatically boost it's strength, allowing it to reach an equal footing with Vasavi Shakti albiet very briefly. It becomes a battle of will from this point on, but since Sieg's opponent was Karna who was famous for his indomitable will, Vasavi Shakti was able to once again begin breaking through Balmung with ease. Before the killing blow, however, he was saved by Astolfo with Achilles' shield Akhilleus Kosmos.
Later, during his fight with Shirou and with no command spells left, an enraged Sieg used Galvanism in tandem with the sword skills he learned during his transformations as Siegfried to fight on somewhat even terms with the heavily injured and one-armed Shirou. He was forced to use a weakened version of Blasted Tree against Shirou in their final fight as his trump card after his sword was broken and he was impaled through the chest.
Also displaying the ability to transform into the dragon Fafnir due to him having Siegfried's heart giving him dragon blood as he used this to carry the Greater Grail to the Reverse side of the world where phantasmal beasts go where no human can venture so the wish for humanity does not occur as phantasmal beasts are invalid targets. As a result of the transformation he lost his physical form and his final state is that of his soul.
The new powers that Sieg gains by his own efforts and the ones the God of Light gave him are these. Sieg must become a master mage and having all the powers of an Average One, such as controlling Fire, Wind, Water, Earth, and is able to use Norse Magic, like Norse Runes, and Gandr and Seidr along with Celtic Magic such as Druidic practices like Celtic Oghaam. Also Sieg after facing his fears and reservations of Alchemy he learns how to master Western and Eastern Alchemy along with all his powers in Fate Grand Order.
Sieg can also change his body to take on Dragon traits from the Type Moon Universe and can take upon his dragon form and shift back to his human form without consequence. His dragon form can also change it's size to have the size of Ancalagon the Black and the ability to create storm s like Ancalagon and his strength and fire power as well and hypnotize people with the Dragon Spell like the legendary Glaurung in the Similrillion. However he can only do this after unlocking his potential through meditation along with his Aura through meditation as well. Sieg's Semblance can be whatever you want it so long as it fits his cannon character and abilities but Dust just gives his body a boost after Dust mutating and transforming his Dragon Form into just gives his body a boost and transforms his Dragon Self the form of a gold and black Western like European Dragon with two long horns on his head with silver hair, two wings, long and muscular arms, body, and legs with a long tail that ends with an arrowhead but with fur on the end of it and with the whiskers of a Chinese Dragon. Sieg also eventually inherits all the skills and physical prowess of the Heroic Spirit Siegfried and his weapon and sword skills and Noble Phantasms becoming a beast on the battle field eventually matching a Heroic Spirit with the physical strength of his dragon self when he transforms parts of his body to have dragon traits and can take full Dragon form without being reduced to just his soul but this takes time.
Part 3: Sieg's love life. Sieg after settling into Remnant and palling around with Team RWBY Sieg begins to fall for Yang Xiao Long and she starts to fall for him. But Sieg is still having feelings for Jean; his girlfriend from Fate Apocrypha. After realizing that the world of Type Moon is long gone and he has a chance for a new beginning he then decides to try and get with Yang Xiao Long which does happen and Sieg becomes a truly great boyfriend and supporting of Yang Xiao Long at her lowest and helps build her back up to her best and the same happens when shit hits the fan with Sieg when he falls into the depression of losing every one he knew in his old life. Yang when she notices that Sieg has shown to be attracted to her she tends to tease Sieg and mess with him but gradually falls for him due to his kindness and refusing to see her as an object or to pursue her for her looks and the fact he is a bit awkward and clumsy but also somewhat wise and intelligent at times.
Part 4: Sieg's personality. Sieg comes into consciousness without anything to form a personality. He is scared of everything but himself, believing to be nothing. He calls himself transparent and colorless because he feels he possesses nothing and had received nothing from his birth. After escaping and receiving the hero Siegfried's heart, he becomes notably more selfless and brave as he gradually continues to develop as a person. Of his own volition, he decides to forsake a life of happiness, and instead aims to fulfill the desperate wishes of freedom his brother and sister homunculi who would die if nothing was done to save them. Eventually, he even willingly becomes a participant of the Holy Grail War in return for their promised freedom and safety as well as the sakes of both Astolfo who had previously saved him and Jeanne who protected him. Due to his young age, he is initially very much naive and clueless about things such as common sense, basic human behavior, and how the world works though is able to pass as an eccentric but otherwise normal, if slightly clumsy, human with a calm and logical personality in a very short amount of time. While typically very polite and humble, Sieg can at times display a blunt attitude during the rare instances he feels annoyed or frustrated. He also has a tendency to blurt out slightly rude observations without really meaning to which occassionally gets him into trouble with others.
He seems to have a high capacity for forgiveness as shown when goes as far as to unhesitatingly throw himself in harm's way to save both Caules and Fiore when their lives are in peril despite the Yggdimillenia's treatment of his brethren. But at the same time Sieg is extremely vengeful when those he cherishes have been harmed before him, not caring whether his enemy is a magus or a Servant, the only thing that matters in his mind is the fact that he will kill them.
After their final encounter with Assassin of Black and witnessing her, along with the children of Scotland Yard's horrific past's, Sieg's innocent belief of humans being slightly flawed but inherently good creatures crumbles, leading him to began to question humanity's true nature. While he takes Ruler's advice on not giving up on humans to heart, and ponders over the opinions of several other heroes on the matter, Sieg ultimately seeks his own answer to how he views humans and life as a whole. Similar to Siegfried, he places little value on his own life and comes to view himself as something that can be used to save and help those around him, causing him to make reckless choices simply for the sake of what he believes 'he should do' instead of what he wants to do. He claims to have no interest in the normal, peaceful life Astolfo spoke of before, but it is revealed during a conversationg with Jeanne that deep down in his he does have a longing for it but willingly overlooked this fact for what he feels is a more worthwhile purpse that can grant meaning to his life as well as honor the hero Siegfried who willingly gave up his life to save the dying Sieg. Thankfully, both Ruler and Laeticia eventually encourage him to start considering his own value more as he realizes that just worrying about the value of humans and his unfortunate sense of obligation towards others without looking into himself would leave him truly empty in the end no matter what he ended up doing with his life.
While Sieg had his own reasons for opposing Amakusa Shirou's wish for salvation, the main reason Sieg chose to trap himself on the Reverse Side to prevent mankind's extinction that would have resulted from the priest's wish at the end naturally wasn't solely for humanity's sake in and of itself. In the first place, Sieg himself admits he hadn't experienced enough to completely believe in humanity though he accepted their nature and simply chose to strive to believe in the good they are capable of like the heroes he came to know such as Astolfo, Chiron, and Karna. But mostly it is because he witnessed Jeanne D'Arc's struggles and eventually came to understand her that he acted. For her who had everything taken from her due to betrayal in life, who received the worst treatment conceivably possible for a human, and yet never cursed anyone in the end and even gave up her life to protect their future, Sieg wanted to 'become her strength' and thus carried out her will, acting for the first time on his own personal desire rather just giving himself for someone else's sake because it's what he felt he should do.
When encountered during the Inheritance of Glory event in Fate Grand Order, he's developed even more. His personality is relatively the same as it was at the end of the Great Holy Grail War, but he's notably wiser and tempered by his past experiences. He is still somewhat awkward at interacting with others by himself and attempts to make himself appear as a solemn dragon, an act he fails comically at as he often has to take many pauses to properly explain the current situation, flounders between the rough pronouns and the more dignified ones when referring to himself multiple times in the same conversation, accidentally joyfully roars loud enough to briefly deafen people when they agree to assist him.