A concept that I've been thinking about is, in short, a 2D sidescrolling team-based, multiplayer Nuclear Throne-type game - you run around in two teams of X players (probably around 3-4, and the maps would be relatively small) in a nuclear wasteland, killing each other with guns and stuff.
In more detail, you have 3 main "mutants"/classes (unmutated humans, frogs and birds), 3 weapon types (ranged, melee, explosive), each having 3 weapons in them (submachine gun, sniper rifle and shotgun; knife, sledgehammer and spear; grenade, cherry bombs and mines) that you can freely mix and match.
Each of the classes and weapons would be unique and distinct from each other (humans are all-arounders, with a double jump; frogs are slow, tanky and have a single, high jump; birds are fast but fragile and can fly) and would have their own stregnths and weaknesses (grenades do decent damage and explode on impact, cherry bombs do a lot of small damage chunks but don't explode on impact (rather after a set duration) and mines are powerful but have to be manually detonated/stepped over by enemies and have a small arming time so you can't "facemine" people). You can only change those before the game starts though, so you have to be wise about it. You could also pick an utility item, such as a portable teleporter or a health pack or a sentry gun, but not sure if that'd be necessary.
Apart from the equipment, you can also choose upgrades for the weapons that will be available for you in the shop in the game (you can select 3 out of X possible upgrades, so you have to choose wisely - upgrades would range from an extended magazine to extra fire rate to explosive bullets, and each weapon would have its own set of upgrades that you could unlock in the campaign mode/through online play) and you can also choose mutations (which are upgrades that affect your character and their effect is far bigger than the weapon upgrades) that you will be able to choose when you gain a level in-game when you kill enough enemies (there could be some neutral/team AI-controlled mutants/monsters). For instance, you could gain a mutation that lets you dash sideways to avoid damage, or gain health back from attacking enemies (lifesteal).
There would be some bog-standard gamemodes like TDM or CTF but also something akin to a MOBA gamemode, where you have to destroy the enemy base while protecting your own, with turrets and all that.
I had plenty of sketches for the characters and weapons in my notebooks (boring lessons are boring, plus I need to practice my art), and eventually I hope to make this game one day. Until then...