I tend to be fan of Faith/Dean as opposed to Buffy/Dean. I have written a few stories that are SPN/BtVS but I have never written one with Buffy.

Those two worlds work so perfectly together. I could have Joe's watcher group simply be a branch of the Buffy watcher's network. That would be an easy way to bridge the three worlds.

Now look what you did... you let the plot bunny out. :-D