While I prefer Djawadi's score, I think that ship has sailed. Too much time has passed, and I don't think Marvel really cares about relationships with anyone, much less composers. Besides, Djawadi was a last-minute replacement when Debney was unavailable (Favreau's usual collaborator).
Debney's score was quite good, though. It took a little while (and the release of the complete score for the film), but I actually really enjoy it now. And for those saying IM2 never had a theme, go listen to the last track on the album, "I Am Iron Man", and tell me that isn't a theme. Yes, it wasn't used very often in the film (from what I recall, it was primarily only used in the fight scenes, such as Monaco and the drone battle at the end). I'm really intrigued by how Debney could further expand and evolve the sounds of Iron Man. While I'm not opposed to multiple composers (I love each of the X-Men scores more or less equally), I just want some consistency, especially in a Marvel Studios franchise.
I also see Debney as the more likely choice, seeing as how they used one of the his IM2 tracks (Sledgehammer, I believe) for practically every Con appearance for The Avengers.
However, given Marvel's track record (with Alan Silvestri being the only returning composer in the studio's history), I'm not entirely confident that he will return. Shane Black might bring on John Ottman, but I'm not sure what their working relationship was, so it might not have clicked like other filmmakers. It might've just been a paycheck for Ottman.
Then again, Favreau is still a producer on the film, so if Debney has an opening in his schedule, he just might sign on.
(By the way, for those of you wondering why Debney was credited with Spider-Man, he was a co-composer on the film, along with Deborah Lurie.)