Do you have any intention of messing with 2 Player Game, or perhaps switching it into a “Luigi mode” or something?
More suggestions that’d be fun if possible:
- Using secret exits (keyholes, etc) in SMW to complete a level, or entering warp zones in SMB1/2J/2USA should unlock a special level in the SMB3 map, such as a coin ship or secret toad house, or give you a rare item (like a cloud)
- If an SMW level is finished with Yoshi, and the next level is an SMB3 one, you get put in a kuribo shoe (like Mario Maker)
- Replace some spades minigames (and/or roulette minigames) on the map with the SMW bonus game (that you get from getting 100 goal tape points) and maybe the SMB2USA roulette too
- Whatever powerup is in the reserve item box in an SMW level is added as an item in the SMB3 map inventory
- Randomize which Airship levels end a world, and include SMW’s “Sunken Ghost Ship” in the mix