Sorry if this brings up bad memories for some, but I just returned to playing Kenshi and Blacklight.
The former is a game where the wild, Wild West meets swords and a strategy game put together, so you manage your squad in the desert by having them loot stuff, sell the loot, train in combat alongside the local policing force, etc. They just added slavers, though you can't really enslave people...yet.
The latter is just recently updated, but still generally consists of ultra modular everything. Your gun's stock, ammo, sights, barrel, muzzle, receiver, paint job, and little accessories can be customized to tailor your play style, though now at the cost of 5$ as opposed to being free. Still not a bad price to pay to be able to unlock everything and everything. Also, it's not just your guns that can be customized. Your character's armor, helmet, tactical gear (which aren't very tactical, but they do change up the elements of the game enough so people want to use the in-game wall-hack EVERYONE CAN USE to try and outflank each other), and even shoes can be changed around to affect your speed, health, and duration of the built-in wall-hack. There's also two mechs you can use, each with a different play style and different weaponry. Some of your gear can add additional capabilities to the mechs.
I also forgot to mention you can use bows, katanas, shurikens, and kunai, although they're all very hard to use in a game about guns, but they're fun if you get to higher levels.