I think it's entirely possible UPN will want a known & proven quality show on their network to give them a bit of respectablity in the years to come. (I mean, have they got anything else on display right now that's even halfway decent?) Seeing as they made their bid for Buffy, I can't imagine them letting it go after a mere two years. It will all depend (obviously) on the actors, Joss & the other writers/directors. If they're getting tired of BTVS & decide 7 years is enough, then that's it. If, on the other hand, they all realize Buffy is something special & an experience not likely to be repeated later in their careers...Well, we shall see in due time. Even if they end the series after S7, they still might reunite from time to time for the occassional Buffy Movie. I wouldn't mind that, myself...Ok, here's my latest crackpot idea for a future big bad: "The Sorcerer".
A youthful-looking but actually extremely old & powerful magician visits Sunndale, and not because of the tourist trade. Even though the Hellmouth has been closed for years, it still attracts all sorts of mystical beings, and this guy has lived so long in part because he drains power from other magic users much the same way that vampires drain blood. And he's come to pick Sunndale clean. (I picture him as dressing like a natty stage magician, complete with cape, tuxedo & top hat! And white gloves, which he removes whenever he's "feeding".)
I see this as a season-long arc, and I think it would start slowly: The land formerly occupied by Sunnydale High has finally been sold for redevelopment, and on the evening before construction is scheduled to begin, our guy (who I see as being played by John Malkovitch or someone like him) visits the still-standing, burned out hulk of the high school, digs through the rubble, and finds the cheerleader trophy that Amy's mother, Catherine, is trapped in. He frees her, and while she's still disoriented, he sucks her dry, killing her in the process & leaving her mummified corpse for the construction workers to find the following morning. (Maybe it's Xander's company, and he's the one to find her?)
As this story arc continues, the Scooby Gang investigates, and slowly find out what's happening & who's responsible, while Sunndale's various magic-users are picked off one by one. Some of them begin to take notice as well, mainly Rack, who's a sleazebag but has an excellent sense of self-preservation. He tries to flee town at first, but by the time he does this, the vampsorcerer has constructed a magical barrier around the city limits to prevent any of his future meals from escaping. Rack then turns to some of his fellow 'citizens' to form some kind of alliance for self-protection, but most of them, being either crazy, stupid or arrogant, aren't interested. (Possibly he visits Doc - who's still alive, natch - and Doc contemptuously turns him down...Only to be the next victim! Oh, the irony!)
Meanwhile, the SG go through a somewhat similar process, and when they realize what's happening, Willow & Tara attempt to leave town as well, only to be blocked by the magical barrier, which also somehow prevents them from contacting the outside world for help. With no possibility for reinforcements, the SG have no choice but to team up with some of Sunnydale's more unsavory elements, and maybe a few ordinary citizens as well. (Like the warlock brother-in-law of the bartender from Beer Bad.)
It's been hinted that Sunnydale isn't the only place on earth that has a Hellmouth (or some similar mystical convergence type deal), so this would be a good plot to finally explore that particular angle & show how many other places like Sunnydale actually exist in the Buffyverse. It would also be an excellent way to bring back (& eliminate) previous little bads like Doc, Rack, D'Hoffryn, Catherine, Amy, et al. Throw in all of Rack's junkie "clients" and the many people we've seen shopping at the Magic Box for supplies to do actual spells (the lady who wanted the Mummy Hand in Life Serial, for one), and there are a lot of potential victims for our evil sorcerer guy. Maybe Dawn's status as the Key could tie into it as well, with her displaying some latent occult power to mark her as a potential target, which would certainly make it a more immediately personal problem for Buffy. And the season climax could be a super-huge mega battle. Whee!