the obvious (sometimes "boring") ones are :
more exp/money per collision
more bounce on hitting wall (or less drag while in air)
throw harder and/or faster
more/less cube mass
more cube(s)
the ones i think would be more fun (mostly when combined) are:
+/- gravity (gives you a slider that increases/decreases gravity, each upgrade makes the max/min values bigger)
-traction (lets the cube slide infinitely when maxed out)
more bounce on hitting wall (but it can go into the "negatives" and the momentum grows each bounce)
cube customization (colors, faces, hats, especially hats )
more spin on throw (cub e, ro ta te)
something with right click? maybe explosions?
not exactly an upgrade but direct control over pos and/or rot values would be epic, no momentum though, that would be OP, i just want to be able to move my cube by exact measurements.
i could probably think of more, but this is likely enough, i hope i didnt give to much.
also the zoom is broken and if you zoom out it doesnt move the cube properly and it gets "moved" up since the center is the same but now the bottom is "closer" so it drops and gives 8-11 exp, you can just zoom in&out quickly to farm exp.
lastly, i love this game, its one of the best time wasters ive seen.