I can't think of anything off the top of my head. I've seen the whole of YYH, but only a few epsiodes and some great fanfiction for BtVS. I have a general idea of how Buffy is and thought she'd make a great partner for Yuusuke (esp post-Angel). He'd really enjoy havning a human around who can keep up with him and not hate him for being hanyou. Keiko doesn't count in my book. She's spririted but not a fighter like Buffy.

If the PTB were some obscure branch of Reikai it would work, I think. After all, We've already seen that Reikai isn't really against using demons when they have to. The only major tweak I can see is that in YYH it's mentioned several times that Hell and Makai are two different places.