I had a proposed idea of TWST AU fanfic crossover with Project Wingman.
Basically, it's the world of Twisted Wonderland had met the apocalypse on all mankind. It took 4 or 5 centuries to rise up mankind again after the Calamity era. There is a new element called "Cordium", where we developed new technologies, where railguns is a common thing. The Twisted Wonderland in the AU is some lands are separated like Briar Valley, Queendom, Shaftlands, etc. Some lands from their nation have declared independence. New sea has been formed and new lands too. Many of the Faes, beastmens, humans, merfolks, etc have witnessed the Calamity and survived the apocalypse.
It sets in 532 AC (After Calamity era), it's basically like modern era. But, post-apocalyptic of Twisted Wonderland. There is one new superpower (like big power composed of several nations.) that was formed on the Coral Sea such as part of Scalding Sands, Land of Heroes, Land of Dawning, part of Queendom of Roses, Sunset Savanna & Shaftlands. This superpower is called Coral Federation (It's like Soviet Union, but larger), they keep expanding by any means of political or military force, they also hold the largest amount of Cordium, which can be used as an energy, or fuel source that can be weaponized.
Reference: https://projectwingman.fandom.com/wiki/Pacific_Federation
Mercenaries is a common thing in this AU, like fighter pilots mercenaries, troops, navy, etc. They're known as "Sicario Mercenary Corps".
The protagonist is still unknown for which I haven't decided yet.
Project Wingman is a flight action video game, that can be played in PC, PS or Xbox. It's a fun game, you will love it.
Anyways, I apologize for this long post because this proposal idea has been stuck on my head for a while now....