I am not the biggest Tokusatsu fan, watched a few Power Ranger series growing up and started reading Marvel's run on Ultraman after Ultraman Rising. So I asked my brother for some information about most of the things that will be discussed, after all he knows his Kamen Riders and even convinced me to watch Shin Kamen Rider during a small family barbecue, so keep in mind these were all his ideas and that I myself don't have encyclopedic knowledge on the subject
Let's start with Power Rangers:
There are two ideas that come to mind, one being Tommy Oliver (green ranger) as the shield hero, having access to his master Morpher to transform into different forms like black dino ranger or red turbo ranger. Now I wonder how the powers of the morphing grid would affect the shield, would it gain a new form from when transforming with Tommy or could the morphing grid overpower the shield's limitation to allow Tommy to pull more offensive moves while being able to inflict damage (also I remember hearing a kickass fansong about Lord Drakkon, but that only showed me how busted the power scaling can be with Rangers as the morphing grind can allow someone to become a reality bending god if they get enough from it)
The other idea involves Kikai Sentai Zenkaiger, where we have the five rangers be sucked into the shield hero world with Juran(red) being the sword, Gaon(yellow) being the shield, Magine(pink) as the bow, Vroon(blue) as the spear and Kaito(white) being brought along with his Geartlinger and some Sentai gears, though you would have to do something as to not have them be stolen since unlike the legendary weapons they aren't physically bound to the users bodies. Since Zenkaiger is a bit more comedic I think you could get away with making the fanfic more silly, with the inhabitants of the shield hero world initially believing that the Kikainoids are simply wearing armor when that's just how their mechanical bodies look. Maybe a bit of plot/arc for season one being that the people of the kingdom (specially the pope, nobles and the king) doubting Kaito's abilities since he is not a legendary hero, with the others sticking to him and pointing out that he is their leader.
As much as separating the heroes is a major point due to the weapons repelling each other, keeping them together since this group of oddballs is better with each other, perhaps giving something akin to a passive for Kaito that would allow that to happen (something like "leader of heroes" or "ranger commander") and that would eliminate other characters like Raphtalia and filo since writing five characters in the same team could be hard and not allow space for other members. Since the legendary weapons don't allow you to wield any other type of weapon, The Kikainoids could absorb their Geartlinger as to gain weapon forms that would allow them to perform most functions and transform into their ranger forms. Now for the Mecha's, perhaps the wave bosses could do the classic move of growing huge after being initially defeated.
Ultraman is a franchise I have dappled a little with as previously mentioned, so I know he is powerful but I can't quite pin point his personality but there's still something to discuss here:
someone already suggested Ultraseven which I should probably watch as to get an idea of how that could work in personality and power wise, The problem is finding it with subtitles as I doubt it has had a dub around my country. There's also an Ultraman with the power to summon a spear and shield made of energy (I think he is from Malaysia, as one of the few international Ultra's in that continuity). There's also been the recent Ultraman rising which let me just say I loved it and Ken is in a mood, and he has a lot of personality to go around in a setting like this but he is also very impulsive which could lead to problems. The main problem is power scaling as Ultraman is quite powerful and the sheer size difference would make him a threat, also how the Shield interacts with Ultra is also to be taken into account as the Marvel version for example has Ultraman inhabit the body of a human together
To Kamen rider... Oh boy where to start? I'll only talk about the series my brother watched and only the main Riders (even if some secondary riders were very cool)
first an idea involving Kamen Rider W, with Shotaro as the shield and Philip as his companion, being sucked in together when Shotaro gets summoned, already the events of the first days change as Philip would be an unexpected factor as a civilian being summoned, with perhaps Shotaro learning some of the heroes strengthening methods and other game things as Philip keeps asking curiously about their games and similarities. Now for the Gaia library, either it could remain the same and only contain information of Shotaro and Philip's world, or it could not work for a bit as it begins to adapt to the shield hero world, though even with the limitation of needing keywords to better find the information needed through the sea of knowledge it could still be broken as hell. Now for their Rider transformation, since it is technically a fusion of sorts it could double their stats or perhaps Philip's side doesn't have the shield's limitation and thus being able to wield weapons. Though considering that every time they transform, Philip's body goes unconscious as he joins Shotaro as a voice in his head with control over one side of his body.
Either by placing it in a safe place guarded by another character or having the shield do similar to the Xtreme Memory and absorb Philip's body and fuse it with Shotaro's body in their Rider Form becoming one in body, mind and soul. Only problem is if their Gaia memories are stolen or lost, you couldn't even write something about the other heroes being given them for them to use and abuse with the possibility of turning into a Dopant since they would need a Connect Shooter, to then produces the USB port-like tattoo on the user's skin called a Living Connector to insert the Gaia memories, having them absorb the gaia memories could honestly be a waste since we don't know how a item like a gaia memories that represent parts of all of Earth's knowledge would affect the weapons and limiting W arsenal just means that there would be less variety in his fights.
Shield hero has lite rpg and MMO game mechanics and system, Ex-aid is literally the video game Rider, and Emu role of a doctor helps with the support and healer role of the shield and his love and knowledge of games could actually put him above the other heroes as perhaps his shield conscious/spirit could implement much more mechanic than the other heroes, though he does have the problem of having his Gamer Driver and Rider Gashats stolen and lost, but Emu probably would have the hindsight to store then in the shields interdimensional storage as to avoid such things. The problem is how the shield interacts with the Kamen rider transformation as we don't know what type of extent the limitations of the shield could affect characters with transformative powers
If going to the route of making a hero whose power is literal to become a Kamen rider through a legendary weapon, a decade or Zi-o like ability to draw power or just straight up transform into a specific Kamen rider would make the most sense. But it does require rewriting the lore of the world. I did have a thought on using the other riders from Zi-o because their designs are so good and their monstrous appearance could be a cause of conflict
these are all he gave me, I don't know if I'll try any of these in the future and the text he sent me was a bit messy so I tried to translate it for your understanding (This was all in my native language of Brazilian Portuguese so I did my best to convert into my self taught English, I would use a app like Grammarly but it has been a bit impractical last year, I don't know if it got better but I can't really see it helping much)
