Personally, I could see Stark giving over plans of a Hulkbuster to General Ross or Nick Fury. Which would then infuriate Bruce because he trusted Stark and sees that Stark, while a good man can also be persuaded by money and can be corrupted even if he is a superhero. So, then you could bring in bits of Civil War and the Ultimate Avengers movie where the team feels like disbanding/fighting within one another, in the end though they realize they need to come together to address the bigger threat---whomever they decide to use as the main villain(s).