Holy shit snacks, this is good news. Just putting it out there: the Slayer-power watered down because it was shared by so many girls, they somehow focus it back into one person, Buffy as a watcher, ASH is head of the watchers with guest episodes, Angel will be hinted at but I can't see him make a comeback. Spike - iirc, didn't he and Illyria team up in the comics or something ( could have been a fanfiction I read tho)? I would love that dynamic being explored. But as much as I would love to have his sarcastic gorgeousness back, it would be a real task to make up a storyline to explain why Spike aged... unless they just choose to ignore it altogether and I would be totally ok with that. Xander I simply cannot see written back into the show.
I do think the show has potential to introduce a ton of new characters and separate from the original storyline, I just hope it will be done slowly and with a lot of loving care. The underlying themes Whedon was so, so good in addressing were important 20 yrs ago (and some of them, like the female power, remain to be so), but if they want to keep that depth, they will need to adjust to current times both in what they choose to address, and how they do it.