Looking at Wikipedia (not always a reliable source, please bare this in mind) it looks as though both Jenny Humphrey (Taylor Momsen) and Eric van der Woodsen (....) are set to return at some point during the remainder of the final season.
Wikipedia also says that Alexa Chung is set to appear as herself and, even more excitingly (is that even a word? Well, it should be!) Cecily von Ziegesar, the author of Gossip Girl, looks set to make an apperace as herself too!
The only downside seems to be that Vanessa Abrams (...) isn't mentioned, but I guess I can deal with that, seeing as Cecily von Ziegesar is!
Cecily von Ziegesar