Okay, well let's expand on that.
S1 I think B/X is definitely a possibility. Xander is a core cast member, and maps over from the Pike character in Joss' original Buffy movie script. He's close to the original conception of things, and I think he's a bit of Joss' personal stand-in on the show. He's the wise cracking outsider male character. The original dynamics have Xander hot for Buffy, Willow pining for Xander and Buffy annoyed with Angel. Angel was only a recurring character early on and was not even set as a vampire when the pilot ran.
Now once Joss hits dramatic paydirt with the Angel/Buffy romance which is pitched for the huge operatic storyline, he runs with it. It comes to fruition in S2 and the show defines itself really in those episodes where Angel goes bad, kills Jenny and Buffy is forced to kill him. It was such rich material and they created the ur-narrative of the show. Much like Dark Phoenix in X-Men or Daredevil/Elektra/Bullseye - that story became the emotional core and touchstone of the series.
By S4, they've got Xander hamstrung narratively. He's peripheral to Buff and the main story. We've discussed before that by sticking with Xander as Everyman, they limited his storylines. But they could have had Xander joining the Initiative, getting the superserum, having Buffy attracted to him as he became more confident and competent and battle-ready. It might have made a more interesting tug on Buffy between the Initiative and her slayer duties. Basically, they could have substituted Xander for Riley in those storylines.
In that same time, if they really wanted to bring Spike back, they could've played off the chemistry between AH and JM. Which was there, and alluded to in "The Initiative."
Just saying, it would've been an interesting way to go in S4.
Or, I could see Buffy and Xander in S7 - they had plenty of old married couple dynamics around taking care of Dawn early in the season. Familiarity and trust gliding into intimacy.