Apparently to be discussed are an SF series (possibly an adaptation of his short story collection Tuf Voyaging, which he has hinted several times recently might be in development), a couple of historical dramas and the potential Game of Thrones prequel series, based on his Dunk and Egg short stories. If any of these come to fruition at HBO remains to be seen.
Meanwhile, there was also a lengthy discussion of the series featuring Martin, showrunners David Benioff and D.B. Weiss and a large number of castmembers. Some interesting tidbits were dropped during the interview, with Martin indicating that his initial discussions with Benioff and Weiss happened whilst there were still only three books out (dating it to before A Feast for Crows' publication in October 2005). They then pitched the show to both HBO and Showtime in March 2006, a lot earlier than previously believed (HBO optioned the books in January 2007).