First thing that comes to mind is Toejam and Earl 2, where they've got to can and collect the humans and ship them off the planet. It would be cool to see this concept expanded with maybe, like, a weight station at the end where you have to see how much trash you gathered? Right now the portions of the game (collecting, shooting) seem a bit disconnected.
I think it needs above all something of consequence, e.g. an oxygen tank where you can only swim for so long, just to provide a sense of urgency, but there are some solid starting points here!