I think it would be cool if players had a reason to return to levels once they have more abilities. There could be objectives that can only be accomplished with certain skills. Obviously we’d need to have a way to not confuse basic players with those goals when they are unachievable, maybe if the goals changed based on the items that the player has in his inventory. Then on the map screen there could be an indication if all currently achievable goals had been met. Or maybe they aren’t actually goals that ever show up, just things that are on the map but unreachable so a player has to remember to return to the level. (Possibly a note when you first gain an ability that allows a repeat about the possibility to improve an earlier solution.) Here are a few examples that I thought of:
The construction hammer could start with only the ability to build fences. Then at some point you pick up instructions for the fire-trap that you equip along with the hammer, or possibly it is just an improved hammer. Winding Trail & Kithgard Gates are examples where the hero can escape by just building a fence, but later return to build fire-traps and actually defeat the enemies and earn the bonus. I’m picturing maybe having a hero in a cage in Kithgard Gates so if you defeat all the enemies you unlock a new hero.
Another use of fire-traps could maybe be to blow up a section of a wall opening a passage to something.
I could also see maybe some of the really early levels having a spot that you can’t reach with simple movements, but once you get the moveXY boots you can get to them. (or if you realize that you can use moveRight(3).
Oh, and maybe an enemy on an early level that you don’t know the name so the only way you can kill him is by having the glasses to get the name of the nearest enemy. For example on “True-Names” there could be a bonus objective hidden behind the Dungeon Door that requires killing a nameless enemy.
Ooo, what about a level that has an enemy that is across a chasm so he can only be killed from range, or even a target that when hit from range drops a bridge so a player comes back when he has a bow.
I guess probably most levels could have some form of a hidden objective that gives players a reason to return once they have more abilities.