I just discovered your recs (they were pimped in the comments of a dueSouth fic finder community) and thought I would just let you know how much I'm already coming to love them. I also thought I'd let you know that there already has been a FMA/Supernatural crossover (http://ishnaru.livejournal.com/44603.html). As far as I remember, there are hits of Elricest but the actual sex is between Ed and Dean.

As for bad fannish ideas, I would love to see a crossover between Hellblazer and Buffy where John Constantine and Ethan Rayne meet. Of course, that's probably largely due to the fact that I am such a sucker for crossovers.