I'm not sure about this, so if there is a copyright lawyer present just speak up, but, I think that copyrights have a shelf life. After a certain number of years it becomes public domain and no additional fees are required. Again, I could be wrong about that since, contrary to my own belief, I don't know everything.
Unfortunately, thanks in part to our dearly departed late congressman Sonny Bono, and the work of corporations such as Disney in particular, public domain was pretty much effectively irrelevant for the last two generations. In fact, the "Sonny Bono Copyright Term Extension Act" was nicknamed the "Mickey Mouse Protection" act because of Disney's massive support of it to preserve the character of Mickey who would have entered the public domain not long after.
What's interesting is that - all those term extensions are on the cusp of expiring, again. Unless they pull another Act and extend it even further (the public outcry would be much larger now than it was then, largely due to the Internet), we face a situation where after 2019 they will start expiring again, and given the age of Mickey Mouse he will be one of the first to go into public domain.
And in this case, there is no one to "sign a deal with" to lock them up - you can't "buy" the rights to something that legally belongs in the public domain. So we face a situation where Disney may no longer "own" Mickey in the not-too-distant future - they wouldn't be prevented from using him, but neither would anyone else be prevented as he would belong to public domain.
(It's funny, because as I said, public domain has effectively ceased to exist for the last 40 years, the concept of a corporation not having perpetual ownership in something is a foreign concept to most folks. It's a big reality that folks are going to be dealing with - over the next decade or two.)
There are a lot of complexities to it - technically, the earliest Mickey films will enter public domain, first - which means Disney would still have copyright protection for, say, Mickey's Christmas Carol, but their rights to the character itself are based upon those original shorts. So, it's sort of like how anyone can make an Oz film because WoO is in the public domain - but they cannot use story elements such as the Ruby Slippers without permission because those were a product of the 1939 film and copyrighted by the current owners of that.
What will happen to Mickey will likely be what just almost happened with Superman (although that wasn't about Public Domain quite yet - they basically were fighting for control between now and then) - where it would have come down to what issues of the comic introduced what concept (say, when did he first use X-ray vision, when did the Daily Planet get introduced, etc.) and what contract was considered legally binding at the time of each element's introduction.
It was one of the messiest cases in litigation history, but to boil it down we could have ended up with a situation where one party owns certain elements of the character, and another party owns others (for Mickey, that would be, say, you can use the concepts presented in Steamboat Willie as that would be public domain, but he couldn't have a dog named Pluto because Pluto didn't appear until a couple of years later, and would remain owned by Disney until his first appearance became Public domain). In any case, it leads to a point around the middle of this century that Disney doesn't own pretty much anything, LOL.
I am absolutely sure that Disney already has folks working on this, and has been for quite some time. My guess is that they will try to separate what we consider "Mickey Mouse" into multiple characters for copyright purposes, trying to say there is a "vintage" and "modern" Mickey - so anyone can then make a T-shirt of old-style black-eyes Mickey, but they would still retain ownership of the more modern white-eye Mickey. That's not how it's intended to work, they would have to prove quite a case - but as I said, I'm sure Disney's lawyers have already been working towards this and that's the only plausible route they go - the public just will not accept a further extension which would be really absurd at this point anyway.