Well some of the characters that i would like to see in a fighting Fe game are not that "popular" but well here i go:
First i would like to see a variety of shapeshifters like Tiki, Nowi, Panne,Kaden and Keaton *especially Keaton* etc.
Marth, Ike and Micaiah, Sothe, Ephraim, Eirika, Roy, Eliwood,Hector, Lyn, Leif, Corrin, Robin, Chrom,Roy are some of the lords and main characters that come in mind, (idk about lucina, i feel she would be just a clone, also the lords of fe5 which i don't remember their names)
For side characters i can name a few but keep in mind that i have't played all FE games:
I would like to see Canas, Henry, Jakob, Cherche, Joshua, Titania, Frederick, Gaius, Emmeryn would be funny too, Felicia (as a Jakob alt) Artur and Arthur,Beruka, Nyx, Amelia, Saleh, Seth, Inigo, Owain, Brady, Myrrh
Obviusly i'm missing a lot.
For some of these characters to get a chance of being in the game they should add a support option so they appear and help the fighter
I would like to see the Hoshidan and Nohrian royal family (yes even Elise and Sakura punching stuff)
For Villians i can think of : Lyon, the dark knight, Gangrel, Zephiel, Garon
Well i think that's it but i'm not that good making rosters and i'm missing a lot of people oh well...