For who I'd choose, throwing logic entirely out the window and any restrictions regarding importance, I'd have to have 5 slots.
1. I've always had a thing for female swordmasters, namely Lyndis, Karla, Marisa and Mia, though what I've seen and heard of her, Arya seems pretty awesome as well. I'd have to go with Lyn on this one, though any would be a fine choice. And while she's not a swordmaster, Eirika also deserves a mention.
2. The Hero class is also another of my favourite classes from FE, though I've had little experince with it due to there only being 3 in the games I played (unless you count Ike, who seems to be somewhat of a Lord version of a Hero). Both Raven and Gerik stood out though and made a firm placement in my army. Again, I think I'd have to go with Ike with this one, simply as he suits the power based style the Hero class offers, as seems to have made ranged swords his thing.
3. Never been much for Axe users personally, but Lance users are some of my favourite. The main ones that come to mind are Ephraim, Nephenee and the wyvern riders (mainly Heatn, Cormag and Jill). However, as much as I like the Wyvern riders, I fel their mounts should be incorporated as part of their moveset, but I just can't see a ood way of implementing their profession into their attacks. Both Ephraim and Nephenee however can handle the job themselves just fine, and either would be a good addition.
4. Mages seem to few and far in between for me. I generally use a lot of them, but character and aesthetics wise few appeal to me. Really the only ones I like a lot are Pent, Lucius, Soren and Micaiah(from what I've seen of her) and somewhat Sanaki. Going to go with Micaiah on this one, mainly because she interests me the most.
5. Notice how I didn't mention any dark magic users above. As much as I had fun with Canas and as awesome Pelleas (GoD) looks like he's going to be, I'm going to have to include Lyon instead. A villain character that becomes playable at some point, we get to see how Lyon would work as a character. He comes with his own Dark magic tome, and I think his necromancing abilities give him a possible unique trait. That and I personally like his background and story. An honorary mention goes out to Zephiel though, despite not me playing FE6, but from what I've read about him and seen in battle, he seems to be pretty badass.
However, realistically, the only character I mentioned I expect to see is Ike. I feel Micaiah has somewhat of a shot, but Lyndis and Ephraim will have to struggle somewhat to get in. Lyon unfortunately seems to have no chance despite how much I'd like him to.