In my opinion, Buffy is way too recent to reboot. There is way too much potential for divisive content, rather than the (mostly) uplifting and positive message of the BtVs. I just don't really see how it could be done better than it was.
It's mostly because Buffy is a cultural icon at this point. An uplifting, positive role model that has become immutable in the psyche of her followers. Not that there couldn't be a slayer (or bunch of them) that had similar qualities and messaging that happened to be a different color, but Buffy Summers is the stuff of legend.
Those of us that grew up with characters don't like to see them recast.. Dell, when they had to recast Dumbledore after Sir Richard's death between HP2 and 3, there was a tremendous amount of outrage, and that was only going from old white guy to old white guy. It was just such a huge difference in the presentation and the portrayal that people were jarred out of enjoyment. (to be fair, Michael Gambon did eventually warm into the role and win over most of the fanbase.)
I'd be more interested in a show simply called "Slayer" that established new characters in the same world as the original. Whether in the comic continuation timeline, or in a post-7 earth B type of situation. Then you give the opportunity of cameo's by characters from series 1, as well as a way to build on the foundation that was created. Like others have said, build on the mythology that has been created; don't just throw it in the trash! (See also: DC CU after throwing out Nolan-Batman)
Personally, I'd like to see the original series given a real HD or 4K remaster treatment in the original 4:3 ratio; not the garbage scan that was on the streaming services for a brief period. You know, the one that converted to widescreen, screwed up the color temperatures, and cut out key parts of the show because of some dialog. (They yanked it because it was so widely condemned).
Instead spend the 20-50 million needed to make the original look amazing and you don't need to spend 500,000 on a reboot that will likely bomb.