Sad that we lost hero mode. I know this was supposed to be more of an exercise than a true polished game, but regardless it sits somewhere in my top ten games of all time. I love it for its nuance, variety, and ambition. I really hope it regains accessability and doesn't become lost media, as that would be the death of a truly great project.
There needs to be some form of enemies. Also a rock form that can walk through fire and swim through acid. Water needs to be a part of the game and acid being water only accesible by certain forms. Spikes that fall from the ceiling could be good and trip mines(like ground spikes but once they kill you they dissapear). A fire form may work for something. Different animal forms may work and a jetpack or glider for long jumps. Hope this helps!