The basics of this idea are that the player can create their own level using everything in levels currently (Pillars, spikes, saws. etc) and then test it, maybe save them for later use?
More in-depth things in this mode:
Customising player's upgrades: Maybe give the player upgrades so that when they play the level, they have the bow or kick, etc.
Chaining levels together: The author can say, "this level occurs once the player beats that other level I made," or something.
Rectangle Resizing: The player can put down a small rectangle, then click and drag on one corner to change the size.
Upgrading Bots: The player can put down a swordsman (sword bot?), then rightclick and set it to Mark III if they're a terrible person. Same w/ Jetpack Bots, archers, and Spidertron. Oh, and hammerbots, those monsters.
Jump Pad Control: The author can change where the jump pad will take the player by dragging a target icon. If the icon is on the pad, the player can change the height somehow.
So yeah, any other ideas on this, put them below and if we get lucky the almighty Doborog will take heed.