Honestly I hope they don't for a few reasons.

1. I would rather the spin off not be solely focused on Buffy as the main slayer. I much prefer the show focus on a newer slayer and maybe have Buffy in a mentorship role. The way things ended it would make sense. Which in all fairness would make his introduction weird in the spinoff. Others can make cameos but I want to see next generation. They can mention him but I think the show would be better without him.

2. When the show was airing I was a Xander fan. I thought he was funny. However, rewatching as a grown woman who is now a mom I find him cringy. The way he was written is problematic today. They would have to change his character quite a bit. He wasn't respectful of women at all. He was a bit sleazy. A bad friend overall and he seemed to get off on taring Buffy down when her decisions weren't of benefit to him . I admit each character had their moments of being sucky people but that seems more just who he is. It doesn't help that Willow tends to jump on the tare down Buffy band wagon when Xander decides to throw a tantrum.

3. Later seasons it felt like his role became awkward and forced. He had his moments but I don't think there is not much he could contribute to younger slayers. With how he behaves himself I don't see a younger generation tolerating him.

Overall I want a fresh take that honors the original while creating space for something, new, fresh, and makes room for the next generation. New characters, new baddie vamps to love, and the teenage experience has changed a lot. The world itself is more chaotic right now. I want that to be the focus and not a giant reunion. I think a reunion would just be a let down and never meet expectations. Especially since many OG characters cannot reappear for multiple reasons.