I don’t know if anybody has suggested this before, but here I go.
My suggestion is to make an X-Com mod in which the events of the game take place in 1898, or sometime before the Great War, maybe with some (we’re not talking to stick gears and goggles to everything, we’re talking serious) Steampunk.
Instead of boasting assault rifles and laser weaponry, the 1898 X-Com soldiers begin the game with single-action revolvers and lever-action rifles (think Colt 1851 Navy revolvers and Winchester rifles) and, as research progresses, they adopt double-barreled shotguns, double-action revolvers and semiautomatic pistols (Webley Mark IV or VI revolvers and Mauser C96, Steyr 1912 or Luger P08 pistols), bolt-action rifles and semiautomatic ones (Gewehr 98 and Fusil Automatique Modèle 1917 rifles), lever-action and bullpup shotguns (Winchester 1887 and the Winchester 1897 Trench Gun). For the strongest troopers, machine guns from the era (Maxim, Vickers, Lewis and Hotchkiss MGs. It is impossible for a single man to carry and fire those, but it’s either that or parapets acting as unmovable HWPs) and mortars (also weighing a ton).
HWPs are armoured cars or tanks of that era (something like a shortened Mark British tanks) flamethrowers and gas throwers (as HWPs, essentially wheeled, 2-ton canisters or even as “backpack” canisters for your heavies) or even howitzers from the time (significantly “snubbed” to fit the 2x2 tile space). For more advanced guns, Tesla-esque stuff or adapting the Heat-Ray from Wells’ Tripods or rayguns and whatnot.
Armor, in the form of your basic coveralls, full WWI soldier outfit, which provides very slight protection and, mainly, a bonus to Bravery, bullet-proof vests (I think they were developed in the 1920s, so they fit in context) and some kind of Steampunk or early Dieselpunk power armour.
As other equipment, possibly the very rifles I mentioned before, bayoneted (as in, they get a Snap Shot, an Aimed Shot and a “hit” command, but that limited and unlimited ammo thing sounds odd. Whatever.), cattle prods, to capture aliens (no idea if they were in use at that time), cavalry and officer swords (Drive me closer, I want to hit them with my sword!), and your standard daggers and grenades, in many fun shapes and sizes, baseball bats for the lulz, field medical kits, flares...
On the alien side, instead of our well-known Sectiods and Ethereals, the X-Com-nauts (or whatever) fight against the Hither and Thither people (from the Lieutenant Gullivar Jones book), the Barsoomians (the John Carter saga), be it on foot, mounted in their steeds or with their steeds doubling as Reapers, the Tripods from Wells’ War of the Worlds, the Selenites from The First Men on the Moon and/or from the early film A Trip to The Moon, and the Séronis from Out of the Silent Planet (and whose description mysteriously resembles the race the Tripods use as food... coincidence?). If I’m not mistaken, I recall the Séronis having some kind of “telekinetic powers”, so there you got your Psionics and your new Ethereals. I’m not an expert on science fiction of that time, so I’m sure many other alien races could be found. Pre-Soviet or early Soviet sci-fi come to mind.
How would the interception and troop deployment mechanics? No idea. While airborne warfare began in WWI, paratroopers or airborne troop carriers were not used until WWII. Maybe the aliens arrive instead crashing in cylinders, like in the War of the Worlds. So, you only get Alien Bases (crash sites) and Terror Sites (crash site in a city). Just an (unlikely) option.
I haven’t the slightest idea about pixel art or programming or OXC’s engine, so effectively this is just a sterile idea. I’ll leave it here, waiting for a benevolent soul to come...
