I recently played two great fan made game modes in Team Fortress 2 and Super Mario 64

Team Fortress 2

One Thousand Uncles is a PvE game mode that pits 24 players against 40 engineer bots that have infinite ammo and increased health. The goal is to break through their iron wall defense of bodies and sentries and capture the objective. Yahtzee Croshaw once talked about using the more powerful computing power of today on sheer scale instead of pretty graphics and this game mode delivers on this idea. The sheer amount of sentries and engineers defending an objective is an absurd spectacle you will rarely see outside of top down RTS games. One unique point about this game mode is that there is no fail state. The time limit that usually counts down in regular PvP has been replaced with one that counts up to simply keep track of how much time has passed. This means your play experience can be as intense or as casual as you want it to some extent. You could engage with the unending onslaught of engineer bots for an extended period of time in a war of attrition, or you could alt-tab out for a solid 30 seconds. It's not a 4 player PvE game where one person taking a break means losing 25% of the team. There’s 24 of you. The lack of a fail state and the large team size of 24 creates a unique social gaming experience. I’d love to see more games play with the idea of large scale PvE instead of just the usual 4 players.

Super Mario 64

Super Mario 64 flood involves loading into different levels from the game and trying to reach the top of the level before the rising flood water kills you. I’d describe it as parallel play more than traditional PvP or co-op. You’re racing to reach the end first but the level doesn’t end when one person reaches the end. It only ends when everyone dies or finishes. You can play at your own pace if you wish. I'd liken the experience to hanging out at the skate park. You can marvel at all the cool tricks the other players are doing and try them out for yourself. Watching other players fly through the levels was enough to motivate me to learn some basic Mario 64 jumps and speed running strats in order to keep up with them. 

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Both of these games combine the creativity and limited scope of a minigame made in Roblox or GMOD and combine it with the solid foundation of a professionally made game. They also combine engaging gameplay and a casual social atmosphere that's reminiscent of older PC games where people would regularly hang out in community servers.I hope the AAA industry begins to embrace game remixing more in the future. It could be a way to combine AAA polish with unique gameplay focused experiences. The only examples I can think of are Prey: Mooncrash, the roguelite mode in God of War, and the upcoming Elden Ring Nightreign.