One problem I have a lot is the wall option. When creating the wall, if your mouse goes diagonal, it leaves tiny gaps were powders can get through. A suggestion to this could be to make a 'straight line' tool, 'connected line' tool, and a 'normal line'. the straight line could draw a fixed line that goes either vertically or horizontally, the connected line could be the same as at the moment, only it connects between the small gaps, and normal line could be exactely the same as what you have.
Another thing you could do is have an option to have downward gravity, witch would pull the solids towards the ground.
You could also make a magnet element, but other than that, the game's great:D
'straight line': Line snapping was added recently, hold Alt while drawing a line.
downward gravity: Moving solids have been suggested many times before. We know people want them, but TPT simulates everything as individual particles. It doesn't do so well at simulating particles connected together as a solid block. Not likely to happen any time soon, sorry.
Also, it's a good idea to use the search bar and read previous threads (e.g. about magnets and moving solids) before posting.