Here are some suggestions i wish to propose. Some of them have already been suggested, but i will put them in one thread.
So here they are:

- Circle tool (already suggested)
- Invert/flip tool (i find it a pain to do one half of something, then having to do the other half exact same.)
- Different erasers, gas, liquid solid etc. (already suggested)
- Rotate Tool (could be useful? select a part, then rotate it)
- a line tool that only does 0, 45, 90, 135, 180 degrees etc, so only at 45 degree angles.

Feel free to criticize, just don't be too smart.