"A few of the main cast, and that's it"? The only characters from the main cast not returning would be Anya and Spike, and I'm not certain that Spike would have not been there if a Season 8 weren't planned. Then there's Faith, who could have a whole story arc centered around whether she should return to prison or stay and help train the new Slayers who would be popping up all over the place, and then there's her relationship with Buffy and the gang. Would she begin to fit in, or remain an outsider?
Xander's grief over losing Anya would have to be dealt with, and the Willow/Kennedy thing would have to be worked out one way or another.
Giles would probably not be around that much, what with rebuilding the Watchers' Council, and Andrew would probably be working with him.
Wood was in pretty bad shape after the final battle, so his recovery and his relationship with Faith would be in there somewhere. Do the Potentials from Season 7 stay to be trained, or go away, either to their own duty stations or to help recover other new Slayers ? Buffy and Dawn would have to face being literally homeless, and the issue of whether Hank would finally accept his responsibilities as a father would come into play.
Spike and Buffy would have a whole set of personal issues to work out if the "I love you" scene were left in, considering the fact that his sacrifice would have been somehow avoided.
And then, just to add spice to the mix, the whole crew would have to set up a whole new base of operations ,but then, with the fact that there were a Season 8, there might still be a Sunnydale...
Aside from all of those issues, there would be the small point of the season's Big Bad and the yearly Apocalypse...
Yeah, you're right. Joss had written himself into a corner there with the ending of Season 7. He would be hard pressed to find anything to write about for Season 8. I guess we're lucky it ended where it did, since there just wasn't any more story to tell...
Oh, I've got an idea. How about we have some sort of love triangle with Spike, Buffy, and Faith, then we could rekindle the feud between Wood and Spike, with the added fuel to the flame complication that Wood's now jealous of Spike for breaking up his cuddle fest with Faith, so he starts up a relationship with Rona and gets busted for child molestation, then he makes a bargain with some demon master to get revenge, maybe D'Hoffran, Anya's old boss, and they could form an alliance to take over the world...