I like the idea its just that making an enemy more grander than Edelgard and TWSITD and Rhea would mean alot of work on writing a character who's further in control of things than Thales. I think it could be done as well just as others said however it would have to reduce the number of characters that you can use (for example if you were to make it so that Rhea would be one enemy along TWSITD or die characters like Seteth, Flayn Catherine and Cyril wpuld have to be unusable until and even after she is defeated tho Flayn may be an exception from this as she i feel is more willing to change her mind).Another example would be to have Claude flee midway as he believes Byleth can finish the rest and have him take certain character too like Hilda or Lorenz. You could do something similar with Edelgard/Hubert and Dimitri/Dedue Other than that the amount of chapters would have to be expanded to include further battles and story for all characters.
