There were people who were confused about what is happening in the story, I will explain what most people are asking about.
1. Who are the sudden new characters?
From HF Universe, Sakura, Shirou, and Rider are living a peaceful and happy life. One day, a mishap that caused them to die, but before they reached the Root, they were dragged in by the Class Cards who were sent earlier than the Miyuverse cast. The collision diverted their course and made them end up in Illyaverse.
2. Where is Miyuverse cast?
Currently, the Miyuverse cast is still in transit. They will arrive in canon time of Illyaverse. That is why I called this arc Pre-Prillya.
3. Why HF cast?
It's because I wanted characters who would settle down. If I picked Fate Shirou, that would be pretty cruel, reuniting him with the alter version of his lover. If it was UBW Shirou and Rin or even plus Saber on the alternative ending, they would be problematic. Why you ask? UBW Shirou will butt heads with his new family in the Illyaverse, and they won't stay around enough for my liking.
Yeah, I picked HF cast cuz they were a lot easier to integrate to the story and they probably be friendly when hearing about Miyuverse cast circumstances. Can you imagine the friction between Fate/UBW Shirou if they met Miyuverse Shirou? Their difference in ideals will probably make them antagonize each other.
By the way, who wants Rin x Archer? I think I wanna write a scenario where Rin is pretty indecisive on which guy she should choose. Seeing that Archer is available, but not being able to let go of her unrequited love for Shirou who she admired in the past. Trolololol.