Can't say I'm on board with the idea of Iron Man vs Captain America in CA3, it goes against the whole "master plan" of individual movies leading into the group movies.
That being said ... it's almost confirmed that Avengers 2 opens up with Iron Man and the Hulkbuster armour fighting the Hulk in South Africa, and Stark is responsible for the creation of Ultron, so perhaps Avengers 2 will see Stark's ego getting bigger which would then be a catalyst for the Super Human Registration Act.
So they can have their little fight and then come back together to fight Thanos.
Still more enticing than this.
2016: Batman v. Superman: Dawn of Justice.
2016: Suicide Squad
2017: Wonder Woman, starring Gal Gadot
2017: Justice League: Part 1
2018: The Flash, starring Ezra Miller
2018: Aquaman, starring Jason Momoa
2019: Shazam, starring Dwayne Johnson as Black Adam
2019: Justice League: Part 2
2020: Cyborg, starring Ray Fisher
2020: Green Lantern (reboot)