Well, I have a few from the Buffyverse. I thought about a Spike spinoff, but from Buffy S6/7 and Angel S5, most of his ground is already covered. We didn't really get that with Faith, so a series focusing on her is something I'd really be into.
I've always, always wanted a Buffy movie. I'm talking original cast, Whedon-directed, not a reboot. I know the odds are getting longer by the day, but there's a part of me still holding out hope! I would probably want it to be just a Buffy movie, but with Angel and Spike as big parts of it as well. Considering most of the Angel cast is dead by the end of the series, they're really all you need. I know it would be a little hard to structure (explaining Illyria to people not all that familiar with the show would be pretty difficult), but I think they could build off "Not Fade Away" pretty well. I always found a bit of a plot hole with that episode with Buffy not being there. Behind the scenes I get it, but just from a story perspective it takes a pretty big suspension of disbelief to think that Angel and Spike are about to die and she's not going to do everything in her power to save them.
I'd like more Firefly projects as well. Not necessarily a spin off, but I'd love it if they kept producing new movies.
Smallville always seemed like a series with the potential for spinoffs. The CW is producing a new Green Arrow show this fall, but I'm not really sure if it's connected to the Oliver Queen we saw on Smallville (it's a new actor, so I'm thinking no). There was talk, years ago, of a Bruce Wayne/Batman spinoff, but because of the blockbuster success of the movie franchise it got nixed and Bruce Wayne never appeared on Smallville (which is such a shame). It might be a little late now, but with the movies seemingly wrapped it would be the perfect time for a Batman series, focused on the early years and when he's training (kind of in the same mold as Smallville with Superman). DC will want to keep producing Batman projects, he's too valuable a property to just sit on the shelf.
I'm sure there are more but those are the ones I can think of right now.
