The only one on the list that I don't agree with is the Star Trek and Mass Effect one. No matter how hard I try, I just can't see that one going well. In my head they're just too different on too many levels.
And that Resident Evil and Silent Hill crossover...at first I thought it was a bad idea. But then I sat and thought about it, and it grew on me. If that was done right, I think it could be the shot in the arm that both series need (sadly, Silent Hill is dead now, but let's just pretend).
I mean, all the RE characters are hardened killing machines at this point. They can stare giant monsters in the face and smirk while they spout out one-liners. The concept of 'fear' doesn't exist for them anymore. But throw them into Silent Hill, and I'm pretty sure these character would start falling apart. It would be great to see Leon actually terrified, or Chris booking it with a real sense of "Oh crap, I'm going to die!" for a change. The RE characters have seen some serious stuff now, and I'm sure Silent Hill would have no problem playing around in their heads. It would be tricky though, because making them too weak would be an insult to them and RE, but making it so they can just gun everything down would just turn it into another RE game.
Disagree on the Wesker and Pyramid Head part though. It would be neat to see Wesker gloat, run up and punch Pyramid Head with all his might, only to have PH not even flinch, and then we finally get to see Wesker go, "Oh no..." before he's violently killed, but I wouldn't want to see a drawn out fight between these two. It'd be too much I think.