I have nothing against a MU/Avatar, but I have some desires if that is the case for Fire Emblem IF:
The Avatar either is the main character and doesn't overshadow the real main character (Like FE7's Mark)
If the Avatar is a Lord, which I think would be awesome, I don't want class selection or anything like that, I just want to be able to choose my weapon and growth rates/base stats.
I want the Avatar to have a consistent personality. I dunno how to explain this but Robin in Awakening seemed so... fake? I dunno it seems kinda out of place for an amnesiac tactical genius to know how to beat several different forces of enemies, but not win in chess. (I get it was a sort of don't want to lose anybody thing, but I mean its chess, he should have been able to win.)
I also hope that if there is a MU, he isn't basically a walking god who everybody wants to get with and kiss at his/her feet because without them all would be lost. (I mean yeah, it would be lost since the avatar is supposed to be you, but still.)
Anyway that is just my opinion on what I hope for in the Avatar for Fire Emblem IF, assuming there is one, there might not be. Until I know for certain I'm definitely gonna look forward to any new information we receive.