7. Wolverine and Captain America
We all know Wolverines old. With his regeneration ability he could potentially live forever, but he wasnt too old when WWII rolled around. Many classic storylines chronicle his enlisting in the military and both meeting and fighting alongside Captain America. The first one even shows them rescuing the Black Widow. A great use of this crossover would be in a Captain America sequel. They could either use the WWII storylines or just reinterpret them for modern day, seeing as how there seems to be no gaps left for their meeting in the original movie. They could even be rescuing Black Widow, making for an even better crossover, and setting up Wolverine for making an appearance in a future Avengers movie, seeing as how he did join the team at one point in the comics.