It'll be interesting to see if Disney tries to stop it. If they do (maybe via trademark [which "should" fail]), it'll mean more publicity. If they don't, then the film will probably quietly make a bit of money and we'll see people playing around with more old characters in new lights.
I'm not super into mickey mouse, but the huge variety of rather enjoyable Sherlock Holmes parodies tells me that characters and stories becoming part of the public domain will be a net good for culture.
There's already a famous case where someone parodied Micky Mouse and lost. Basically, if the parody is too much like the original you can lose, and it's not fun fighting Disney's lawyers on how much is too much:
I'm not super into mickey mouse, but the huge variety of rather enjoyable Sherlock Holmes parodies tells me that characters and stories becoming part of the public domain will be a net good for culture.