I think Xander's endgame for Buffy. They DO seem really comfortable with each other, you guys are right. There is this attitude out there that that level of comfort is not conducive to passion or great emotion and that it is somehow incestuous, but I very strongly disagree. Most of the people, including myself, whom I know to be happily married have a very strong friendly/familial bond. Passion is necessary in any romantic relationship, but it's not the most important part of a functional relationship. The friendship/family bond... that's the one that gets you through illnesses and career setbacks and deaths in the family and other long-haul type problems. Xander is already sort of taking on the co-parent role with Dawn, the confidant role with Buffy, the second-in-command role with slayerdom, etc. etc. He seems to be growing into Buffy's partner. Who better for a lover than a partner?
Also, I strongly suspect that Xander will be freed up by the mole. I'm not sure about the Buffy/Satsu thing, but I don't at this point really see it being a big romantic ship. It seems more like experimentation, which actually makes me sort of mad at Buffy. I mean, she knows Satsu loves her. Like someone else said, it's like Spike all over again (only I hope she's at least less violent with this one). That's another thing... I don't think Xander would stand for that. Buffy seems to respect him as an equal. Even the slayers junior seem to respect him and have faith in his experience. I think a Bander relationship would be a lot more healthy and balanced than most of Buffy's other ships... same for Xander, really.
Xander and Buffy have been leaning on each other a lot for all of the little day-to-day stuff and the emotional bonding ever since season 7. I think that once they wrap their minds around the fact that feeling close like that doesn't make sex taboo, they'll be good to go. Actually, rather than being incestuous and icky, I think that close family/friendship relationship has the potential to make the sexual part of the relationship much more satisfying. Think about it. We recently saw some performance anxiety on Buffy's side, right? She enjoyed her romp with Satsu, but judging by her comments afterward, she was also passive and insecure and unsure of herself. That's not the case when you are already close to someone before you start a sexual relationship with that person. In the already close situation, you have fewer inhibitions, so you feel free to communicate your needs and desires... which results in having them met as well as having confidence that you have met the needs and desires of the other party. So the result is a lack of panicked declarations of "but I didn't do enough things!"
So, basically, I am pretty firmly in the Bander camp. I think they could work really well on all levels. Plus hey! No need for that embarrassing sexual past talk, since they both already know!