If the planned reboot was anything to go by, Buffy would be black.
I think they would also make Buffy a lesbian and just have Willow be the "ugly duckling" best friend who is actually really pretty once she takes her glasses off and dresses up a little bit (such an annoying trope that Hollywood loves).
Buffy would probably be a bit of a Mary-Sue that changes bad boy Angel/Spike's ways with her love if she's not a lesbian. If she is then Faith will be the bad girl who has her ways changed by Buffy after opening Buffy's eyes to new horizons. It would be as cliche as hell.
Xander would either get killed off or, as the object of Willow's affection, realise at the end of the series what was in front of him the entire time once she's glammed up a bit.
Giles would be a woman because you can't have an adult man hanging around with underage girls or he'd be "non-binary" or whatever made up gender is flavour of the month.
Riley would be a woman. Buffy might be bisexual so that they can really play up a love triangle.
Dawn would be just awful because of what they'd turn her into.
Joyce would either be super supportive of a lesbian Buffy or would absolutely hate her guts but Buffy's minor status (at least in the early seasons) would prevent her from leaving.
Whedon would not be involved.
Episodes would lecture the audience on issues (like what Doctor Who does ever since Chibnall took over).
There would be no women or POC or LGBT in The Watchers Council, it would be exclusively made up of straight white men to represent "straight white male privilege".
Kennedy would get a bigger role and become an official scooby.
Xander would be treated as a damsel in distress that needs saving every week.