2 parts.
1 - Put two characters that have never or rarely interacted into a team up. What's the common case or reason for them meeting/teaming, who are they fighting and whats it all about?
2 - Similar to the above but not a one off. Two diverse MU characters meet and hit it off during their team up forming a lasting friendship or partnership perhaps ala Power Man and Iron Fist. Who and how would you do it?
The only rule is that they previously had no or minimum interaction. So you can't have say Hawkeye and Hercules but you could have Hawkeye and Blue Shield.