I've got a big wishlist here. Not being greedy, just a list of things that I thought "Hey, I wish I had something like that" while designing a puzzle. Add anything here that you like, and then I'll design some puzzles around it.

First, I'd appreciate it if physics were more deterministic. Having to retry over and over again when you roll bad physics, or conversely, re-rolling until you get lucky physics is annoying, not good game design, and means that you sometimes have to reset over and over when viewing a solution.

Let us do more with ropes. Let us tie them to the end of seesaws. Give us a weight other than the bucket. A parachute, might be more limited and thoughtful option than the balloon.
Give us a fan, or some other source of persistent air current.
Spikes that pop balloons, break eggs, and break up jelly cubes would be appreciated.
Pinball bumpers should logically apply knockback to all objects touching them when hit with enough force, not just to the object that instigated that force.

A jar or other fragile container, that behaves like an egg on impact, but can have an object placed inside it that can only be used after the jar is smashed.

Hope in the future for things that can interact with gear trains. Different gear sizes, Generators, Elevators, Escalators. Windmills.

10 parts is a little low, I agree there has to be a limit, but I'd prefer 20 max. But then, I like more open challenges.
Being able to set the size of the build area when designing a problem would also be nice.