The thing about Joss - I think that he'd want to see a lot of money put forward for the project if he were to take it on, in order to have the creative freedom to do it the way he'd like to. I think he said as much at the Paleyfest (was it five years ago, already?). The VM effort may generate enough funds to get the project greenlighted by the studio, but I'm not convinced that they will have what they need to bring back enough of the old cast members and to create a movie that would satisfy the series fans.
As for BtVS, I just don't know. It's been such a long time - the show ended almost ten years ago! The actors have moved on and also have changed physically. Alyson Hannigan on How I Met Your Mother is an example. Yes, she's always looked younger than her age and I suppose she still does, but she's approaching 40 now and I don't know about her playing a college age Willow. Michelle Trachtenberg is in her late twenties - certainly not a teenager. The older actors such as Anthony Stewart Head may have changed less over the decade but he certainly did look older in his role as Uther in Merlin than he did as Giles.