Please tell me that I am not the only one in here who wouldn't mind seeing The Resistance from Twilight Princess in battle. I've always felt like their designs were wasted, and they never got the chance to reach their full potential in TP. I mean, they could obviously fight, as suggested throughout the game—particularly the entrance to the final battle at Hyrule Castle, but that's about it. I could say the same thing about most of Ocarina of Time's Seven Sages (someone mentioned Darunia). The only problem I can see potentially coming from this is the fact that it'd take up four/five/or more character slots, and that probably wouldn't sit well with everyone—especially those who would rather have some other, more important icons be added to the roster.
I wouldn't want all five of them, unless the roster reached about 50 characters, because it would be too much TP. However, I would like to see Ashei and Auru playable. The two of them specifically have something going for them already. Ashei being a warrior, and Auru having his bog bazooka-cannon thingy. Other than them and Zant, and maybe Hero's Shade, I don't really want too much more from TP.
Again though, that depends how many characters are from other games. For instance, if we got all 7 sages from Ocarina of Time (we already have 2) + Sheik and Koume-Kotake, as well as Tetra, Daphnes, Medli, Makar, Aryll and Orca from Wind Waker, and Ghirahim, Groose, Demise, Gaepora, Fi and Batreaux from Skyward Sword, I'd be far more accepting. That would put us at 33 characters right there. And then there would also be spots that I'd want for other characters like Happy Mask Salesman, Ganondorf, Vaati, Veran, Onox, Captain Linebeck, Byrne, the three goddesses, Tingle, Cia, Valga, Wizzro, Agahnim, Phantom Ganon, Dark Link, Chancellor Cole, Yuga, Fierce Deity Link and Hilda. That would be 54 characters though, which I don't see us getting. xD However, if there is still a decent amount from other games, I wouldn't mind being all 5 members of The Resistance.