4. Natasha Romanoff & Melinda May
Although these two kick-ass females have probably met off screen (what with them both having very strong associations with S.H.I.E.L.D), it hasn't happened on screen yet - and that's something that simply has to happen at some point. They would make an awesome, sexy, bad-ass duo if they were to team-up (seeing them in tandem beating the hell out of a group of male henchmen would be a sight to behold), but seeing them going at it against each other is what we all really want to see. Each of them has displayed martial arts skills that are unrivalled by anyone else that we've seen so far in the Marvel Cinematic Universe and a face-off between the two would be fantastic. Hell, even seeing the two training together would be great. Get this done, Marvel!