This request is inspired by Eternal Red's "The Truce" update, which is absolutely one of my favorite Spuffy AUs. It's an alternate universe in the real sense of the word.
It's really unique because it crosses into a neighboring genre (still sci-fi/fantasy, as opposed to AUs that are AHs, say, pirate/viking/regency/etc.) that is quite different, but retains all the canon-ish elements we all know and love.

Do you guys know of others? Thanks.

I too love The Truce and look forward to each update. Sadly I can't think of any other fics like this. There are fics in other dimensions or in crossovers but the characters are our canon BtVS and/or AtS characters elsewhere, not the characters as completely different people.