Topic: Environmental hazards
Maybe it would be fun if the levels had some sort of terrain or obstacles that could cause damage to your tee, for instance spikes, lava or toxic green goo. This would probably make it a lot easier to create interesting maps, where powerups and weapons are located in places that are a little trickier to reach. Imagine being forced to swing above a lava pool to be able to reach the rocket launcher..
Another thing that probably wouldn't take to much work is slippery surfaces? Maybe it won't matter as much, since you'll always be able to use your ninja rope, but a section of a map with slippery surfaces might be combined with walls and ceilings where the grapple won't stick. Then you would be forced to rely on basic movements.