They confirmed Zevran won't be unfortunately, though his voice actor's moved up a rank to Inquisitor (the American male voice).
Sten can't be in it, because there is no Sten. There is only Arishok 
Isabela stands a good chance because...well, the devs are in love with her. She's moved up from being a one-off character in the first game to a full-fledged companion in the second, and a major character in the comics. As of the comics, she's a close friend of both Varric and King Alistair, both confirmed for DA3. Also, they seemed to want to develop the arc of her past - and she was considering changing her name to Naishe, her birth name.
It'd be nice if we're seeing Hawke if their romanced companion has a cameo too. Though given how romances were given short shrift in Awakening (they had to patch in letters from the character's partner) I wouldn't count on it.
I'm guessing the Eluvians are going to be important, and so is Elvish history, so Merrill would be a useful person. The canon origin - the Dalish Warden starts with that mirror, Origins' last DLC ends with it, and a lot of cryptic messages about Flemeth and Morrigan. The Masked Empire confirms there's a whole network of Eluvians beneath Orlais, which the elves have re-awakened. Merrill is our canon expert on those things. It's notable that you can get to the end of DA2 with a dead Anders, an enslaved Fenris, an imprisoned Isabela, and a dead sibling, but Varric,Aveline, and Merrill will always make it through. That could mean something.