what if we added a big chunk of code to TPT that could quickly add new powders to a still-running powdertoy, you would just write a new powder into a txt file or something, put it in the Plugins folder, and it would add it to the game even if it is still running.
this would be hard to implement, and developers might have to take other plugins into consideration as well since their new element would have to interact with others in some way, but despite this, it would be a fun way to mess around with new custom powder types, and if you don't like the change, you can just move the txt file out of the plugins folder
I would suggest an ini/xml file that gets parsed at startup. The game comes with some standard elements, and you can create new ones based on inheritance. (For example, inherit clone, but make it melt at 600 degrees.) Should be fun, but on the other hand, bit of a waste of time.
I think they're making something like that when they do that source code rewrite, for windows they will or might make some special dll folder for people to put their plugins in. I don't know about mac though !
I think a language similar to XML would be the way to go with this. It would allow you to just run your .exe again instead of re-compiling and would give you your changes.