1.) Easily recognisable video game characters like Super Mario or Sonic the Hedgehog
2.) Castle / Fantasy, use bright colours and magic is always implied.
3.) Jungle themed?
4.) Nether themed with corresponding colour pallet. Use blocks like nether wart block, magma and nether brick. Use nether portals, you could use them as portals to the minigames.
I just threw a bunch of ideas together, so I'm not expecting for any of them to be ideal.