Already ingame?
Lucina. I have the Spring! Brave! and Masked! variants already, somehow I don't have the actual one.
Yet to be released?
FE6/7: Marcus (Gotta add another Jagen, right? right??), Legault (c'mon, let's make Nino happy.), Sonia (or not).
FE13: Morgan (she's a riot), Gangrel (surely there's some archetype he can satistfy), Panne (though this is more in the vein of needing a Laguz type of character in general)