"OMG! Maybe the "real world" wasn't real after all??!?!"?. Gosh, that never would have occurred to me.
What you need is a completely off-the-wall way to end it that becomes obvious only in retrospect. Which, I'll admit, is Hard to pull off, but then there's a reason I don't make movies.
(e.g., having Mal be right about everything and actually alive in the next level out would have been marginally less lame)
Still, both Brazil and The Usual Suspects manage to leave this one in the dust. Hell, even Verhoeven's Total Recall was less of a waste of time. Buh.