Did you ever find the theme you were looking for?
If not, I kind of like @Big Boi 's underground fantasy idea, and I kind of like the post-apocalyptic thing... maybe you've got a skirmish/warband conflict between gangs of troglodytic mutant creatures at endless war in the bottomless shelters, silos, and labyrinthine tunnels and caverns beneath the surface of a bombed-out radioactive ruin of a world?
Elevator pitch: "D&D's Underdark meets Fallout and Mad Max in a deadly Post-Apocalyptic Hollow Earth!"
Could be a good way to make additional use of Wargames Atlantic's fantasy, WWI, WWII, and weird world war kits, swapping out the rifles and machine guns for swords and axes and spears. I recently had a lot of fun converting Oathmark goblin slaves into post-apocalyptic vault-dwellers by swapping gas masks and rifles onto goblin bodies, it would have been just as easy and fun to use fantasy weapons instead, and I'm sure that WA's world war figures in gas masks would work great with a stash of spare fantasy weapons for a post-apocalyptic fantasy setting. Come to think of it, Death Fields bands like the Bulldogs, Grognards, and so on would fit in great, too. The lizard people with their fantasy guns and gas masks might be right at home in such a setting, and the spiders would work, too. Toss in some of the wealth of assorted Cthulhu monster models out there, along with some of the weirdest fantasy dungeon monsters to use as radio-active mutants - gelatinous cubes, beholders, etc. - and there'll be no shortage of off-the-shelf minis to work with. Any of the goblins, orcs, drow elves, and other routine Underdark villains would be easy to integrate into a post-apocalyptic setting, too.
Swap out the usual fantasy treasures for crates of rations, drums of fuel, cases of medicine, and caches of weird weapons of war, and dress up the traditional dungeon dressing with heavy vault doors, elevators, generators, laboratories, underground greenhouses or mushroom farms, and the like - that sort of thing could make some distinctive battlefields for illustrations! Depending on how technological the pre-apocalyptic backstory is, you can toss in some retro-robot "golems", armored tractors, and other souped-up diesel-punk and sci-fi war machines to back up the war bands, too.
For inspiration to mix things up even more versus the traditional fantasy underdark/darklands setting's tropes, you could check out pretty much any of the great, cynical, bleak '70s apocalyptic/dystopian sci-fi movies: The Road Warrior, Mad Max, Logan's Run, Soylent Green, Rollerball, The Omega Man, Beneath the Planet of the Apes, West World, Future World, The Stepford Wives, Dawn of the Dead, THX-1138, Damnation Alley, A Boy and His Dog, Zardoz, Silent Running, Death Race 2000... and, the 1960 version of H.G. Wells' The Time Machine is a great adaptation of one of the great-grand-daddies of both underdark horror/fantasy and post-apocalyptic science fiction, fitting right in with the general theme - what sort of violent hijinks would those savage Morlocks get into under ground when they aren't busy snacking on Eloi? War with each other, war with their mutant neighbors, war with rogue technology and monsters even more terrible and weird and mysterious than themselves, dredged up from the darkest pits of the inner earth... and this is the story of those wars!
