There's no trailer yet, but in the latest Famitsu, they have some updated information on the roster in Hyrule Warriors, and Fi, Link's assistant/sword spirit in Skyward Sword, is playable. You know, her:
The article also states that Ghirahim is in the game, but didn't specify playability. The same article also notes that there will be alternate costumes for Link and Zelda based on their Skyward Sword looks.
EDIT: And here's a trailer for Fi!
She's got some fancy kicks.