There's only a few characters where I have a strong opinion about their inclusion. In order of how enthusiastic I am (and, luckily enough, just about in order of likelihood too, I reckon).
1) Little Mac - Probably the only character where I'll be actually disappointed if he doesn't show up. He's such an iconic NES character, and I mean absolutely iconic. I see Punch-Out!! referenced all the time in mainstream media (didn't it even come up on Family Guy once?). And then there's
THIS bit of Zac Gorman art that's just so perfectly Nintendo... Mac's the only character there not to show up. He's just absolutely gotta be in there.
2) Mii - I know a lot of people just see the Mii as a generic avatar but, I've made this argument before. I play Wii Fit and StreetPass pretty much every day, and I play with my Mii. I log on to the WiiU and that little face is there. I see my Mii on Nintendo hardware more than I do Mario at this point... they're the face of modern Nintendo with HUGE moveset potential. I'd probably be disappointed to see them left out, too, to be honest.
3) Ridley - Or Anthony Higgs. I just want a friend for Samus.
4) Dixie - Donkey Kong is a franchise that needs a new character badly, and while I see the arguments for K. Rool, I'm pulling for Dixie first. She's just so, again, iconic to me. The ponytail, the helicoptering, the pink top, the little beret, it's just such a classic look to me. I'd be happy to see them both get in, though.
5) Mona - Ever since Ashley was confirmed out I've been thinking about the Wario series and, man, isn't it weird that it's only at one character? Between Land and Ware and those other little releases they're at 15+ games, same ballpark as Zelda. I like the idea of adding another rep there and Mona'd be my choice (though Captain Syrup would get me equally excited, I think). Probably not happening though.
6) Robin - Lots of potential here, and easily the most interesting choice for a new Fire Emblem character. Really hoping he/she makes it over Chrom.