While I'd love to see a "Buffy" movie, what could they do? the one thing that limited the time that Buffy could go on was how they set up the big bads- each one was more powerfull every year. They went from:
The Master
To Angelus/Spike/Dru
To The Mayor/Pure Demon
to Adam/Gov
to Glory the Hellgod
They then took a break to become depressing, giving us the Troika and ultimately Dark Willow
then they went all out and gave us the uber-bad First Evil.
If they do a movie, who would be the big bad? Dark Willow maybe, but she seems to be gone thanks to the slayer activation spell( the white hair symbolizing Willow being purified) and the fact that she has Kennedy to support her. Maybe the First again, but I'd doubt it. Wolfram and Hart? Could be.
But again, I hope they do have a movie. There are still things they can do (wrapping up Spike-Buffy, Slayer Acadamey, etc). but I doubt we'll see anything like that anytime soon.