I still have to watch Firefly, i only watched Serenity but with the success of Avengers i bet that he can finally make that sequel he planed. There were many projects he had planed that didn't happen because of money, like a Spike movie, the Reaper tv show and many other projects, he should try to use the Avengers success to promote himself while the success is still fresh.
I like his Angel and Buffy comic book continuations but i would like to see a return to that universe in live-action, i still have to read Fray but it would be a good option. And CW right now seems like a good place for these shows, unlike with when the channel was called WB and cancelled Angel before its time.