Here's what I've come up with with just a little bit of thought:
Post-apocalyptic: Put the Underworld beneath a Gamma World or Mutant Future. The monsters become weird mutants or alien incursions. The Chaos becomes literal radiation, or some reality warping residue of the biggest super-weapon the ancients had. Or the malfunctioning drive of an immense alien saucer. Whatever.
Call of Cthulhu or similar Pulp Horror: Tweak the tone a bit, and Jason's Underworld is every bit as much a lurid place of weird menace as K'n-Yan or red-litten Yoth. It already Great Old Ones and their cults lurking around, too, though it would be easiest enough to substitute known mythos horrors like Eihort or Tsathaggua. Turning lurid up to ultraviolet, I invite you to contemplate the potential parallels with the Shaver Mysteries.
Superhero: This will seem the most unlikely of these suggestions, and it certainly won't be for all campaigns, but I would point to the Silver Age strangeness of Cave Carson (recently rebutted for modern psychedelic strangeness) and even the Mole Man and the various subterranean cultures of the Marvel Universe. Obviously, the parameters of the mission might be different, and the previous force that cleared the path the PCs followed, might will be the campaigns next major villain. (Or the original X-men to your team's New X-Men, if you get my Giant-Sized X-Men #1 drift). You might also want to Kirby up the monsters a bit, too.