Here's a little brainstorming for a Lord of the Rings type of campaign (take the magical object back to it's maker in hopes of freeing the oppressed) for a Gamma World or Mutant Future setting. Here's the analogs:
Macguffin: No ring here, just a super-sciencey/sorcery object (okay, a ring! Or make it a tiara for all I care....) that imbues it's owner with invisibility, while mutating them over time into bullywugs. Oh, and it's psionically controlled by an evil techno wizard who's the ultimate mutant (a giant fiery eyeball!) in a high techno tower.
The mission is thought by the adventurers to be one of subterfuge to take the device back to the mutant wizard's tower to use against him. Unfortunately, he's got other plans...
You: Diminutive hominid of your choice, but can't be more than 50% mutant, I'd go with some form of hare or coney. I believe they call them "hoops" in G.W.
Elves: Grens - slender, greenfolk who are excellent archers that like to shoot first, berate later.
Dwarves: Stocky-figured, Gamma World Badders (badger people). 'Nuff said.
Humans: Valids (pure strains) who are suspicious and yet crave redemption
Orcs: Cyborna (cyborg jerkwads)
Trolls: Golthans - sad, childlike, and under the Eye's control
Mordor: Tall, ebony, techno tower of spire built amongst the ruins of a once great city
Minas Tirith: Gleaming city of platinum, built into a mountain side and barely touched by the cataclysm
Helms Deep: Underground keep of the centauroids
Moria: Badder mines that have been invaded and appropriated by Cyborna
Shire: Burrow holes in a grassy, reclaimed prairie
Smaug: Monster of your choice, but has to be big and intelligent. Why not make it easy and just go with an cyborg T-rex with artificial intelligence? Actually, Smaug needs to be more of a force of nature so perhaps something like an a bio-armored serpent the size of an aircraft carrier.
Sauron (aka "The Eye") is a human who's delving into alien overlord tech mutated him beyond recognition--body, mind, and soul.
Gandalf: Techno wizard who left the order to join the rebellion, loves tech from the ancients but also knows enough to fear it
Saruman: Another wizard, but he's in the apprenticeship of The Eye
Gollum: former hare of the burrow (hobbit of the shire) now a full-on bullywug creep who longs to get the device back in hopes of turning himself normal or disappearing forever....
I can see a long slog through irradiated junkyards, swamps, and foothills. Maybe even use the old Gamma World module found in the GM screen --it's already got a spaceport tower in a mesa-filled wasteland!