I think a single season of Tales of the Hellmouth. This could be the stuff goes on that Buffy isn't involved in, which includes the unanswered questions, like the mantis eggs. There could be some creative ones, like Marcie is sent back (it's her home turf after all) to assassinate someone trying to expose the truth, but she gets attacked by a werewolf (who track by scent), and for awhile at least there's an invisible werewolf running amok.
It could provide insight into other minor characters like Snyder (perhaps before a principal), what the SPD ran into (and why they were inclined to cut Buffy some breaks, but also why when Buffy resisted she was automatically described as very dangerous--they had to know, just as those who awarded her Class Protector knew something was up with her, and it explains so much, including resistance to Mayor Wilkins wanting a Slayer up for Murder One, and perhaps THAT story could be showcased as well).
I'm really curious about the coach who meets his demise in Go Fish. He didn't bother with a gun for the nurse, but did for Buffy, presumably a smaller teenage girl (he also just tried to club Xander more than use a gun). Yeah, he knew something was up with her and wasn't taking chances. Plus the whole X-Files sounding speech of failed Soviet Union retroviral genetic experiments that they failed to accomplish, but he cracked it. I bet he knew all sorts of interesting people (he might've even had a guest role managing Marcie while she was in Sunnydale). There's no way someone of his knowledge was just an ordinary coach. I'm sure there's a back story there somewhere.
How about when the school blew up? That and with tales of graduation horror, would certainly up black ops investigation and containment (and who knows what that might've released from the Hellmouth, perhaps even a weakened witch no longer a trophy...). The Initiative that seemed built way too fast (since they just weren't shown in any real capacity before s4) could've confiscated another college cult secret grounds, or found or compelled demons to move much of that dirt, perhaps with an ominous they don't have to pay them back, they'll collect in their own way later.
They could even throw in a special Wishverse episode or two, either of the one we saw where Xander and Willow being made minions to the Master are shown, or perhaps a different wishverse.
That all sounds awesome. One nitpick though, Catherine was still trapped after the school blew up. Her statue from what I've read appears in a cameo in Doomed. Willow apparently steps on it. I've not seen it on any rewatch though I must admit.
We can only hope that when Sunnydale collapsed she was finally freed/died. She was an evil bitch, but that's undoubtedly one of the most horrifying fates anyone got in the series.
As for which spin off I'd like to see, I think Faith and Spike would have been cool. They are my two favourite characters, had good chemistry and could have been on the run together.
I wish though that they had done it after S1 of Angel and S4 of Buffy. They should have had Spike go on the run after the initiative was dissolved, as after all he had government tech in his head that they wouldn't want to fall into the wrong hands. Faith could have broken out of prison and the two could have met whilst on the run, perhaps in a town where Demons and Vamps rule the roost, like the Wishverse, as that would be the only safe place from the human authorities.
Both would then have to team up to survive, (as neither would be welcome by the Vamps and Demons either, a Slayer, and the traitor) along with other "white hat" characters.
To me that show would have been the best bet for Spike and Faith as characters. You could have got the best out of the Spuffy storyline too by having Spike and Faith get together, but unlike with Spuffy it wouldn't be out of character for Faith. I can totally buy that she would be drawn to Spike, and it would be a different type of romance to Bangel, as both would be loose canons.
Ripper also might have been quite cool. Had it been made it may have allowed the Buffyverse to go in new directions, as from the sounds of things Vampires wouldn't have been the main antagonists.
I also desperately still want to see a Fray adaptation with Dana Delorenzo as Fray, and Cameron Monaghan as Harth. I used to think that Robbie Kay would make a great Harth, and he would, but his accent would jar too much with Dana. Cameron who played the Joker in Gotham would be cool.
