4. Don't Make It A Reboot
First and foremost, Buffy the Vampire Slayer doesn't need a reboot. Yes, the show itself is already one, after a failed attempt at making a movie version, but the show got almost everything right. Its influence can be seen in numerous shows that have followed its 1997 debut, from fellow The WB hits Veronica Mars and Gilmore Girls to the revived version of Doctor Who, and the series' themes remain as relevant as ever.
Everything about the show more or less holds up (and some of the effects and references just add to its 90s charm), but what's more is that while Buffy is a superhero, she isn't one in the traditional sense. The character isn't (or shouldn't be) a Batman or Spider-Man esque figure who gets rebooted every decade, because a key strength of the show was in telling her story, her struggle, and Sarah Michelle Gellar's portrayal, and it's almost impossible to separate her from that character performance.
If you're rebooting the show with a different actress, different best friends, a different Giles (who would, in this case, be the Alfred to her Batman?), then you're going to lose the essence of what made Buffy - both as a person and a show - so special.
No Sarah Michelle Gellar. No 90s setting. No Nerf Herder. Take all that away, and what's left?