Something that I'd love to see is the ability to swim.
If you don't have the powerup, touching water will kill you. If you have it, it will allow you to slowly sink to the bottom until you hit the bottom, giving different physics.
Of course, there still can be lava and other kinds of waters (acid?) that will continue to kill you, so only one special type of water can be used.
There could be even different powerups for this. One will allow you to get into the water (a skill) but only for a limited time, while another is a diving suit that will allow you to actually stay under water for unlimited time.