I actually wrote Bucky & Cas story on ao3, but that fic ended before comfort part could begin and ends in an angsty note. Loki & Cas, is another pair I want to see interacting. Winchester brothers and Elric brothers from Fullmetal Alchemist. Loki & Hannibal, a dysfunctional doctot-patient relationship. TFW with Star Wars trio, Star Trek trio, and Brooklyn 99 team. Cas & Steve being agressively liberal. I really wanna write long Brooklyn99 x SPN crossover but I'm not ready to commit to it yet.
All of these sound super interesting, though I must confess that I’ve never seen Fullmetal Alchemist. I would definitely like to see a SPN x Brooklyn 99 crossover, though. That sounds like it could get hilarious. And I would love to see Cas and Steve be aggressively liberal. FTW Luke, Leia, and Han x Spock, Bones, and Kirk could get crazy. So many strong personalities. I love it. Oh, or did you mean Finn, Poe, and Rey? Either way, it could get interesting.