I'm no insider, but based on my observations, Buffy has a very complex rights issues. The Movie rights are owned by the Kuzuis, the currently divorced couple that produced and directed the film. The TV rights are owned by Sandollar, Dolly Parton's production company. The rights to the elements, characters and scenario that didn't originate in the film, but rather in the TV show, probably belong to 20th Century Fox/Disney, but that would probably be subject to dispute. Mutant Enemy, and the successors to the WB and UPN probably have some interest in the series, although it's probably silent interest. Warner Media was reported as leasing the film rights at one point.
So, to make a Buffy movie, you need to get the film rights. To make a TV show, you need the TV rights. To use Angel, or Xander or Giles, you also need the rights for the elements from the TV show from Disney, as well probably paying off everyone else involved. And you will probably have to pay off Joss too, just to keep him from going Alan Moore on the project.
Or, you Bing up a YA Urban Fantasy writter, but the film rights for a Buffy like property, and churn out a couple films about that. Again, that's my impression of things.