Okay, so a little while ago I threw out some ideas about doing some WWI gaming. I really like the look of the period: biplane aircraft, clunky tanks covered in rivets, the uniforms, the old cars with wooden spoke wheels. But it's not really a setting conducive to RPG gaming. But on the way home yesterday I thought of a new approach. Many years ago I had a thick paperback book on WWI where each chapter was a sort of "short story" on some event in the war. One of my favorites was about the capture of Fort Douaumont near Verdun. The massive fortress was captured in 1916 by a very small party of combat engineers from the Brandenburg regiment who managed to slip in, sneak around like in a dungeon crawl, round up the small garrison by surprise, and capture the entire fort without firing a shot. It was a major coup for the Germans.
So then I thought, what if you take a WWI battlefield setting, make it more sci-fi/steam-punk, swap the single fort for a huge, sprawling Maginot Line type fortress complex, then add in chemically-mutated zombies, and steam-punk automatons. The PCs are the last survivors of a larger team which got worn down crossing the shell-cratered moonscape of a battlefield outside. Surveying the deadly surface, with random barrages, drifting clouds of poison gas, strafing aricraft, and roving units of zombies, they decide their chances are better going in than going back. They head in, equipped with sapper's equipment like dynamite, headlamps, bolt cutters, entrenching tools, a flame thrower, grenades, etc. and proceed to explore and clear.
I really think the Death Korps of Krieg figures from Games Workshop would be perfect for this type of game--and you could even set it in the WH 40K universe if you really wanted. The fort could be inhabited by orks, or infested with Tyranids, or whatever. I think you'd have two main "classes" for the PCs: sappers (combat engineers) and stosstruppen (special frontline assault troops).
Inside they face numerous deadly threats. There are drifting clouds of different weird gases, some poisonous, some mutagenic, some corrosive, etc. There are automatons of various sizes--you could have a huge garage area in the bottom (with a long ramp to the surface) with a huge, multi-weaponed tank crewed by a thing composed of the merged mutated crew: the "dragon" at the heart of the dungeon. There would be lots of weird gas-mutated zombies, some armed some not. Lots of various mechanical traps, tricks, and puzzles to overcome. They would have to figure out buttons and levers, find starter keys, repair and operate machinery for bridges/lifts/vaults/etc.
There would also be helpful items to find, such as ammunition, sealed containers of food and water, signal flares, explosives, medical supplies, etc. This being a WWI-style military game there wouldn't be much in the way of classic "treasure" but some ideas are a shipment of gold bars in a vault, an ornate gold-embroidered enemy battle flag to be retrieved from a tall exposed pole outside, enemy medals to snag, an officer's gold pocket watch, maps of other fortresses, plans for more effective gas masks, mutagen antidote serum, and chemical burn salve. One thing about a game/campaign of this type is PC mortality. There is no magical healing so the party may be whittled down over time. Adding new PCs down in a fortress-dungeon is problematic. Some ideas are to say that other teams were also sent in and they meet a survivor, the ever-popular freeing of a prisoner who joins the party, or maybe one of the denizens of the fortress joins the party for interesting reasons.
In addition to just exploring and clearing the fortress the PCs' orders include signalling that the fort is taken so that relief troops can push across from friendly lines to occupy it. You can have various means available, each with certain drawbacks. There is a radio room, but the big radio is broken and they must scavenge parts from various places in the fortress (and successfully repair it). Signal flares work, but they have to go outside and climb to the very top of a massive tower if they are to be seen all the way back at friendly lines. There is a coop of mixed messenger pigeons--but will they fly the right way? (The PCs should probably start equipped with a basket with a messenger pigeon--but keeping it alive will be challenging.) They find a field telephone switchboard, but which line leads where and how do you work this thing? Someone may volunteer to be a runner and take a message back (solo suicide adventure). There's a hangar containing a hydrogen-filled observation balloon with signal flags, but the roof doors are jammed.
For DMs there are plenty of photos and diagrams free on the web for Fort Douaumont, Fort Vaux, Fort Moulainville, the Maginot Line, Eben Emael, and others. Those should make for a great set of inspirational materials to start with. Add some imagination and creativity and you're off.