I've havne't seen it and probably never will, since I don't care much for sappy romance movies where both the main characters are as boring a dry paint. But still these two would make that movie a lot more interesting. I prefer weird couples.
I'm not a big fan of "fan pairing" at all but i really enjoy this one for some reasons. I've already favorited a few pieces you've made of it lol. As much as i love The Joker and his relationship with Harley Quinn, I always felt sad for her and thought she doesn't deserve and shouldn't take all his abuses.
Deadpool is somewhat crazy, which Harley seems to dig in a man, but i imagine he would treat her well (much better than "Mister J" at least) and that outside of the killing and crime, they would be like most happy or madly in love couple.