Ho boy, a few ideas I've had:
RPGs
Fort RPG - Probably my oldest RPG idea. Players can form their own nations and place 'fort stones', which are huge floating crystal things. These mark an area in which that nation is allowed to build. (Heed ended up doing something like this with PRPG with flags) It would've had some sort of peace/war system. I wasn't sure on the best way to handle that but basically under the circumstances of war/open pvp, players from other nations would be able to destroy your nation's bricks. Once the fort stone is reduced to 0 HP, they would be given two choices. Let it be completely destroyed, in which, the whole fort would slowly decay, or pay the same price it would cost that nation to place that fort stone to renew it. I scrapped this idea because there's no way to find a good balance between fair and fun in this kind of situation. It would also only be of any interest with a high server population.
Turn Based RPG - A turn based RPG more focused on adventuring than it is grinding. Players would go through a story mode, dungeons, or other dangerous areas. All of which would be co-op. I still wouldn't mind doing this but right now I'm more invested in another idea.
TDMs
Mining TDM - Think infinite mining, but TDM. Players mine their way to getting upgrades for not only their pickaxe, but also weapons and abilities. The base can be damaged by any weapon, but takes significantly more damage from a pickaxe. Of course, in this case, being infinite is not such a good idea. And RNG would be fine, but in order to remain balanced, the generation needs to be mirrored. This could be easily done by generating the map into variables before spawning the dirt/ores.
'Grinding' TDM - Players could select from a variety of sub classes based on their chosen main class. Sub classes could be changed at will, but main class would've either been impossible to switch, or punishing. Each main class had some sort of sub class that can be used to go grind. Whether it be chopping wood, mining rocks, or I forget what else. These resources were used for a variety of upgrades and other things. The problem with this TDM is that it requires like, 6 players minimum to even get started, and more to be interesting.
Loot Hoarders - A TDM about treasure chests. The map has scattered treasure chests. Opening one will cause it to spit a (weighted) random item at you. This could be a weapon, an item like a pill or something, or whatever you want to do with it. I was going to also include passive abilities, which could not be dropped. Max of 3, getting a new one or dying bumps the first one off. The objective of the game is not to kill though. In order for your team to score points, you need to drop items into your team's large chest. Players would drop all items on death and start with a weak low/0 point weapon once they spawn. Maybe only allow the player to carry 3 or so items at a time. I was going to include a carrying capacity passive as well. There's a chance I might still do this, and if it's good enough, I'll release the basic gamemode, and maybe the additional features separately.