At this point, I don't believe Dark Pit to be a playable character on his own.
The best he could be is an alt for Pit, which is what I think it's being teased, but I'm by no means right, though I can't say I'm wrong either. Pit also had a "fallen angel" color in Brawl, which was used as a basis for Dark Pit. (There are a lot of similarities, but there are significant differences as well, notably the color of the eyes and the laurel crown.)
I don't think he's an Assist Trophy. No other ATs have been teased so far, not even ones from Kid Icarus, so I would find it really strange to see Dark Pit getting some sort of special treatment.
As a stage hazard/boss, I won't deny it's a possibility. This could be a situation not unlike the Yellow Devil, after all. It also kind of fits with the whole "crashing Palutena's party" joke if he was a stage hazard on Palutena's Temple. (I'm curious as how would the original Japanese dialogue would directly translate...)
What I'm curious about this situation is what would happen to the aforementioned fallen angel costume, as having Dark Pit and fallen angel Pit on the same screen is bound to create a lot of confusion. Logically, Pit would likely get a new color swap, but it would be disappointing especially considering that color swap was somewhat popular in Brawl.
Anyway, the possibilities all exist: Pit alt, Assist Trophy or stage hazard/boss. In order of likeliness, I'd put Pit alt first, stage hazard/boss second and Assist Trophy third.
All that is left to do is to look forward to when will this be clarified.