The details of the contract basically have MSHI frozen in its current state until 2050. Nothing can really be added or removed unless both parties (Uni and Marvel/Disney) agree to it separately.
It also looks like Disney can't use any Marvel characters at WDW, regardless if it's the cinematic universe interpretation of them.
Here's the original contract for reference. Keep in mind there could be additional addendums that we are not privy to.
http://www.sec.gov/Archives/edgar/data/1262449/000119312510008732/dex1057.htm
My guess is that MSHI stays frozen the way it is until Universal decides to close it (never) or Disney buys Uni out of the contract.
I don't think Disney is shaking in their boots since the comic book versions of the characters aren't the moneymakers anyway. The movie versions are.