People have suggested elements like you suggested many times. They all got rejected because moving solids are a pain to code and it would be very buggy. If you want to have moving solids, The Powder Toy is not for you at the moment. Maybe when The Powder Toy is recoded in a new language (C++), it might be possible to do something like that without it being very buggy. The closest thing we have to moving solids now is stkm and soap. as you can see, they are both kind of buggy. If you want to see some attempts at other moving solids in this game try downloading some mods that have moving solids.