It doesn't need to be romantic (in fact, I'd prefer it not be -- but don't not-recommend because they are linked in this way). I'd just really love to see a scenario where these two very different characters have to deal with one another.
If you have something that pulls Angelverse [which I always consider a subset and linked to Buffyverse, even when Sunnydale's crew is nowhere in evidence], that would be extra-awesome.
Also -- I would LOVE to read about others' dreams where they are a participant in SPN, BTVS, or AtS 'verses.
And, One Last -- I always thought that S7 (assuming they didn't kill Castiel for good, which I thought they should, even though I love Misha and the Dean/Cas ambiguous romance) should have Cas turned human. In my mind, this would be because GOD chose Cas to be more humanized and then share this experience with the other Angels. In my mind, GOD loves us Humans because we tend to question what the f**k he's doing, and he admires that in us. Cas though, due to that free-will clause, went off in a direction he wasn't expecting and *boom*... problem.
I have a way, way future fantasy/story in spanderverse where this is the storyline. I'd LOVE to see this explored by others, because there is no way I'm so unique that this hasn't occurred to somebody else.... Share your recommends. Not just for my sake, but for the few readers I have, too.
C'mon -- we need to promote far more AtS/BTVS/SPN crossover universe interaction!