gotta improve on slope physics, they can cause annoying problems with jumping at times, though i heard slope physics are incredibly hard to program so i can allow it. something i'm confident you can improve on without much issue is the indication of semi solids. i say you should edit them to be darker or lighter to tell them apart from full solids, making it more intuitive.
context: 
an example on how other 2d platformers indicate this stuff: 
not really a big deal, but i thought i could help you improve in some way.