I have thought a lot about this (maybe too much) but i came up with the best choice for a spin off is Illyria.
At firsdt i wanted Faith to be a spinoff, i mean i love the girl she is my all time fav. But we have had a slayer show already. I heard rumours of a spike spinoff, interesting, it was a popular idea, but we have had a souled vampire show already. So ok then willow, she would be cool. But then by the end os s7 she has no where to go, i mean she has become a godess (sort of) where has she got to go from there. she's been the uber bad, been the uber good, been the geeky girl, been hetro, been gay, been cool. No where to go really.
I thought of a dawn spinoff, i know many hated her but i though that there was plenty to tell about her life, her character weasn't so bad by the end of s7. But then that would be like a xander show, a njormal person doing normal, or not so normal things, it kinda lacks a catch.
wes is dead, ripper has a...movie? to be made plus he will always be with or near or so closly involved with buffy she'd have to keep coming and saving him.
Anya's dead
I like kennedy but with any luck she is with willow so...no.
But Illyria, new character, learning the ropes (kind of anyaesque really). She is powerful but not all powerful, i think it would be great.
My secon choice for a spinoff would be gwen, i loved her. she was great. and my third choice Nina. let the werewolves have a show. I know all these c's come from Angel but I think the ones on buffy had plenty of time to play thier stories out. perhaps bring one or some of them into the spinoff but not as the lead.
Whatever will/will not/ may happen I will die is Andrew is in it. Can't stand him, never going to like him, i won't say he should have died but i never want to see him again.