One idea that immediately comes to mind is letting the player switch gravity to be left and right in addition to up and down (like the Run games on coolmathgames). This would definitely add many more possibilities to level design! Another thing you could consider adding would be more obstacles that require the player shooting. Examples of this could be large fans that the player has to shoot to turn off for a time, or phantom platforms that the player has to shoot into existence before being able to stand on them.