A couple of months ago I came up with a campaign idea/concept, a sort of isekai/Meta Game (a game within a game sort of deal) mainly inspired by Solar Opposites C Plot "The Wall".
Essentially the plot of the game would be that a group of player joined a weird DND via like an online LFG, and once they joined and met at the location they all faint. They wake up in a medieval fantasy tavern as their characters, isekai style but every NPC is acting strange... Suddenly the roof gets grabbed by some giant hand, which turns out to belong to the GM who is a witch that shrunk the player and turned them into their characters so that they could play DND. All the NPC are also some random people that she shrunk and turned into characters and the whole map is like a huge series of interconnected terrariums that she as made. At first the game would be quite meta, with the players literally "Larping" while the "in game DM" throws the dices and narrates. I would personally allow metagaming in situations were the PCs are playing because they might know rules and such but they'd need to roll to make sure that the "in game DM" does not noticed that they're meta gaming. Each encounter could have some meta like
Bandits: Give me all your gold or I'll gut you like a chicken!
DM From above: Like a fish
Bandit: *sight* like a fish...
Other situations could be like;
DM: Suddenly a dragon appears! *drops gecko in the terrarium*
Players: *genuinely panicking*
DM: Guys chill. Guys. Guys! I won't let it actually attack you!
Gecko: *grabs some random NPC and shakes them around*
DM: Kiwi! Kiwi let go! LET GO!!
Eventually the game could become about the PCs trying to escape and find help and this becomes like a Borrowers style campaign!
Edit: I just thought of that, alot of the NPCs could have been players that she didn't like, like edgelords or "That Guy" or just people she didn't like. So you could have that random quest giver NPC trying to take the spotlight or the bartender being this overly edgy brooding dude, etc