Next to Link you will get to play with Impa for the first time, which is certainly exciting. But it was mentioned that there were many more playable character (Aonuma was even surprised about how many characters there were), where I have trouble wrapping my head around what characters this could be... Zelda? Groose? Beedle? Scrapper?
Well, what I definitely would like to see in the character roster would be some badass Goron. He could use a hammer (maybe even the Megaton Hammer from Ocarina of Time) as a weapon or simply his fists like Darmani in Majora's Mask. In similar fashion a Zora Warrior could be introduced, fighting with a spear made of fish bones. A Gerudo Warrior would also be nice (and add more boobies, because we know that Team Ninja won't stop at Cia). They could aim for introducing their own six sages in the game, but this time each of them is a strong warrior.
I would also like to see Link as Fierce Deity, just because of the fanservice and it could fit in nicely in a game like Hyrule Warriors, where you fight hundreds of enemies as some overpowered entity.
Also, as far as I know the Warriors games usually let you play with the bad guys as well:
It would be awesome to play with these guys, though the problem here probably lies in your opponents. When you play as Link or Impa you get lots of different enemies from the Zelda series like Bokoblins, Lizalfos, Stalfos or even bosses like King Dodongo or Argarok. But what are you supposed to fight as Cia, Valga, Wizro and maybe Demise? Just lots of Hyrule foot soldiers?
I actually answered my question myself: as one of the bad guys you could fight Goron or Zora armies, even have big enemies like Biggoron or the three dragons from Skyward Sword. Also, there is lots of potential here for the bad guys. You could play as Ghirahim or Dark Link for example.