Pretty much need to be approximately 11 years old, give or take 2 years, non-divisive and friendly. Also, being a lead and not completely helpless helps, with the advantage of voter exposure and likability (a median between healthy adult [not moe] and valueless lolita [too moe]). Weirdest/most divergent one out of the bunch is the doll, but pretty much only because she's a tsundere and a doll.
Looking at this, Tamaki's the type. Clannad's safely out eventually because of course there are concerted efforts from a lot of different directions to put down the annoying "obvious" choices -- like with LS last year.