The only thing the contract says about that sort of approval is that the character are consistent with the style guide. There doesn't appear to be anything in the contract that would prevent Universal from, for example, building an Iron Man ride as long as was consistent with the comic style guide.Realistically this would be an exchange of things that would/could be mutually beneficial. Disney could simply bring a pile of money to the negotiation, but you'd essentially be paying for Universal's 3rd gate.
My understanding of the contract is that Universal would need consent from Disney before they utilize these characters in a different way than the original build of the attractions. So if Universal wanted to build the rumored Avengers E-ticket, Disney would have some say/approval in that process. If anyone can correct that understanding, please do, but that's the leverage Disney would have. They can allow Universal to do one thing as exchange for Disney doing another thing.
"Whenever Marvel has “reasonable” rights for rejection of approval hereunder, the basic criteria to be used by Marvel may include inconsistency with (i) basic story line, (ii) the powers, (iii) basic personality traits, (iv) physical appearance (including clothing or costume), and/or (v) living habitat or environment relating to such character as portrayed in Marvel’s exploitation of such character in comic books or other products for the particular time period being depicted by MCA."