I could also add something which is still sorta lacking in current versions still, although not in-game customizations, but instead some attributes for custom levelset sounds.
Hitting sounds for some elements which still don't really have it. For example, for char elements, whether it's a full group '[char].hitting' or separate elements (char_a.hitting, char_aumlaut.hitting, etc.). When you hit them with the beam, they make no sound, which can hopefully be customizable in the future.
Also, as another example, hitting for fibre optic. Yeah, you're not really "hitting" it, per say, more so I think there could be sounds for it's usage, whether through '.hitting' or some other suffix like... '.passing', maybe?
And speaking of, I feel like slopes rather have no hitting sound or it might be using hitting sound of some other element. And I'm pretty sure you can't customize it's hitting it's sound either.
Strangely, you can't really apply explosion sounds for separate elements, it just all defaults to '[mm_default].exploding'. Whether it's things like laser cannon, McDuffin, cell, cauldron, bomb, gray ball, pacman, or bonus ball.
Clicking sounds for elements which aren't hit by the beam just yet. You can interact with some elements like McDuffin, fuse, rotatable mirrors or bonus balls by just clicking on them when the beam isn't hitting them, which means they can be clicked, but you can apply no sound for that.
Property checkboxes for disabling starting random rotations for DF and MM mirrors, polarizers, crossway polarizers, MM teleporters;
Property checkboxes for separating even or uneven polarizers (so rotating mm_polarizer_1 would only cycle it through '1,3,5,...,15' and rotating mm_polarizer_2 would only cycle it through 2,4,6,...,16 separately);
Property for changing values for ice and amoeba wall melting and growing delay;
Property checkbox for disabling clickability of light ball (bonus orb).
Another strange additional suggestion: possibly steel wall chars for MM engine?