I love this so much! In the future, I'd like to see these: (No need to make these in the game, just an opinion)
- Koopas (This would add more depth as there would be more enemies in the world making it feel more lively).
- Mario Pipes (I noticed in 1-1 that the Mario Pipes were replaced with bricks. On the randomly generated worlds, maybe add pipes you could go down that would lead to the kind of battle when you confront a Hammer Bro in SMB 3? Or you could just make it deeper in the level. Either are equally awesome).
- Powerups (Currently, there only seems to be one kind of Powerup: the 1-UP. Maybe add a mushroom which would restore you back to the form you start in if you've been hit? If you were hit, you'd be Mini-Mario. Other than the mushroom, I'd add a Tanooki Tail powerup as well because sometimes you can't get past enemies without killing yourself for the invincibility).
- A Goal Flag/Star (Maybe add a Goal Flag/Star to the end of levels that would add to your total Star Count/Flag Count. After getting a large quantity of these stars/flags, a red Bowser face would spawn on the mini-map and you would have to fly there. It wouldn't be randomly generated and would be the final battle with Bowser. After beating that you locate Peach and the game ends.
These are just some thoughts and ideas. Overall, it's a great game! :D