Great concept. This is a fantasy staple that I think Magic should have already done, but I imagine that because of the gelatinous cube's strong D&D association, they don't want to cross brands, as Mark Rosewater has touched upon before someplace. Not sure on the execution though. I like that you're trying to have the person-trapped-inside feel, but I think it would read much better and probably play better if it was worded something like this:
Whenever ~ blocks a creature, at end of combat, exile that creature absorbed by ~.
~ gets +2/+2 for each creatured it has absorbed.
Then you may also have to say something about what happens, if anything, to the absorbed creatures when the cube dies. I mean, you do want the creatures to be able to escape, I'm assuming, from the -1/-1 counters thing. Possibly re-enter with a -1/-1 counter?
Anyway, I think this version just seems more simple and elegant, because you don't need to deal with all that clunky text about turning things into enchant creatures and losing abilities. If they're exiled, they can't do anything anyway.