What if Riley hadn’t left?
as long as he did not end up with Buffy, i think he could have added something
What if Spike didn’t exist?
That one would break my heart... but the world goes around without people we can't imagine not having, so... it would be a very different show from season 4 onward.
What if Darla hadn’t staked herself?
Connor would have died. Not sure that would be a good thing, but i don't think the audience had stomach for that... but then again, if they'd found a 'solution' and suddenly magicked Connor out of Darla... then they would have had to deal with an aftermath where there might be a custody battle if Darla remained attached... maybe it might change her fundamentally if she had a human child?
oh, some fascinating ideas arise!!
What if Xander had been gay and not Willow?
Well this one apparently had been considered by the powers that be. I think that Nick Brendon would have been interesting as a gay or bisexual man.
A lot of fanfic has explored him as gay. In one of my major stories i imagine him as bisexual because as he's written in the series I see evidence for him noticing men, but he definitely is interested in women... and his behaviour is not stereotypical straight guy, so bi fits for me. I think it is interesting to see how it changes the trajectory of a character if you change that one thing. So many relationships would not happen and others might... This is the one that has had a lot of exploration in millions of words of fanfic, a lot of it very bad badfic. I like the idea of good explorations.
I don't know that I would like to see a totally straight Willow. For some reason, for the most part she reads well as a lesbian, not bisexual, though a case can be made because of her relationship with Oz. I would have liked to see Willow chastened by what she did to Buffy enough not to continue on the road shew as on and not become the big bad... but that is a digression, so i will shut up now!
What if Buffy had been gay, not Willow?
Well, I guess that one may answer itself.
Faith.
Though I really doubt Buffy or Xander would have been considered acceptable by the suits back then.
You don't think they'd have accepted a homosexual male by season 4 of the series? I doubt they would have allowed anyone to show 'tendencies' in high school back then, but by university, I think they could have considered a gay Xander or a lesbian Buffy. It would have been a bigger deal. The problem with Buffy is that her character is almost relentlessly heterosexual. The only time you ever see any same sex chemistry is between her and Faith, but even then, it's mostly on Faith's part.