Suggestion 1: Project Wingman + Ace Combat
Setting: Post-apocalyptic after Yellowstone erupts and fundamentally reshapes the world geopolitically and geographically. Most of the planet now relies on geothermal power and a new power source known as 'Cordium' that was pushed up from the Earth's mantle after the devastation.
Mechanically: Mostly offbrand aircraft with a couple of superplanes. Large focus on Cordium-propelled Airships, that are basically flying warships. Railgun technology advanced enough to be fit on planes and a variety of Cordium-based superweapons that, while powerful, have a chance of triggering geothermal events and volcanic eruptions - possibly up to and including a second world-shattering Calamity.
Important in-universe events - Original Calamity from Yellowstone's Eruption, and a second calamity caused by misuse of cordium-laced missiles that lead to the entire pacific ring of fire erupting. Violent Dissolution of a huge mercenary nation based in Australia, formerly ruled by a council of Mercenary Lords who attempted to control conflict around the world in order to profit from it.
Bonus -
Ace Combat - While I do not believe the setting to be worthy of full inclusion, as a 'units only' series of sorts. There's a wide variety of ridiculous superweapons throughout the series, and interest has been expressed for a muv-luv crossover of some kind?
Suggestion 2: Command and Conquer - Tiberium Timeline
Setting:
Can vary depending on precise timeline point. But to approximate canon - after the end of the cold war (or a "hot war" between Europe and Russia, depending on canon) attempts to rebuild society are stifled by the mysterious growth of an alien fungus in areas around the world. Toxic to human life but also holding huge quantities of valuable minerals and resources, this substance named 'Tiberium' becomes the flashpoint for a series of wars. But even as the world wages war with itself over control of Tiberium, the mineral itself is ever growing. Adapting.
Evolving.
It soon mutates beings it comes into contact with, and eventually evolves to become a crystal instead of a plant. One that consumes and adapts almost anything it touches, spreading like a growth onto the world. And with increasing numbers of unusual sightings it soon becomes clear that Tiberium is not natural to earth. But who placed it there? And will they come seeking their creation?
Mechanically:
There's a lot to choose from here, even before we get to the Aliens. GDI technology is a mix of upgraded tanks and aircraft with a futuristic aesthetic and full-on piloted mechs - generally referred to as Battle Walkers. NOD uses an eclectric mix of out-of-date weapons and adapted superscience, with a heavy focus on laser technology and biological weapons that utilise Tiberium for increased strength.
There are also a few 'Ace' machines available (The Mammoth MK.2 from Tiberium Sun, Redeemer Walker from Kane's Wrath, and theoretically any PC could function well with an Avatar or Juggernaut. )
The Scrin (or player-created equivalent) have a traditional alien invader aestheic, with a dramatic focus on air power mixed in with stereotypically 'Alien' biological ground forces. And Tripods. Who doesn't want to fight giant tripod mechs?
Notes:
I'm ignoring Tiberium Twilight,
and you should too.
Red alert also offer some mechanical inspiration, but the time travel plot felt a little too silly to justify. At least for me.
Suggestion 3: Advance Wars
Setting:
Primarily Days of ruin Setting-wise. Aftermath of a meteor storm blots out the sun and wipes out a great deal of human civilisation, abruptly ending an all-out war between the two previously dominant nations. Scientific Research and Arms manufacturing company distributes a bioweapon as a virus and continues to arm the survivors for their own data - and their own amusement.
Mechanically: While I'm ignoring the first three games from a setting perspective, Mechanically there's a
lot of interesting weird to source here. I personally have an interest in the Great Owl and Owl's nest (
can you see the theme) from Days of Ruin, but Black hole in AW2/DS (mostly the latter) have a variety of interesting technologies that could be adapted for a villainous faction - From the traditional Black Cannons/Lasers/Bombs/Factories to Dual Strike's more Esoteric Black Crystals, Obelisks, and Oozium. Also I'm pretty sure everyone here could write a more threatening villain than oldman man and his lightning-shooting wheelchair.