Talking to Digital Spy during promotion for Dragonball: Evolution, Marsters talked about the last time someone had tried to bring Buffy back:
A couple of years ago Joss [Whedon] came to me because they were thinking of doing a television movie...but ultimately it didn't come through. I think basically Buffy has always been an uphill battle. From the very beginning it was swimming against the stream. It's hard to get the suits to commit to another Buffy project, especially one that doesn't have Buffy in it as the lead.
That doesn't mean that he's not available, however:
Yeah [I would return], I mean, any time you can get that kind of writing in your mouth... [The struggle with producers] makes me just want to write my own and produce it.
I have to admit, I have no idea what a James Marsters-written Buffy would be like, but I'd be more than willing to find out, if only to keep him from having to make another Dragonball movie.
Marsters doubtful about 'Buffy' return [Digital Spy]