I agree with marcgfx and Udderdude.
I'd like to add some more ideas to the pool:
The hole grows, yay. Let's say you have a scoring system. Make a way to delay the hole's growth (perhapes reduce it's mass?) through a kinda difficult 'quest'. This way you have the illusion of the ability to gain score infinitely; but why not make these quests increasingly difficult to accomplish? That certainly adds to replayability; just don't overdo the difficulty.
Not that? Well how about a hidden "evil karma side thingy"? Side with the hole, feeding as many things to it as possible, but not dying either. Perhapes the hole could promise to spare you?
Thanks for the ideas, but in the end Ive decided to go with some different ideas, I just dont think it works as I thought it would, unless the world is infinite, but thats a project for another day when i know more about that