Build and destroy mode should be usable tools with Q, a modified Hammer and Brick Placer. Jet to switch can be kept.
Some kind of message as to which kind of block you're using would help. Create several bricks and give them uiNames for the menu but make selecting any of them use the 4x cube as normal.
Ideas for some blocks:
Valve: Water cannot go through it, but players can. Perhaps replacing the Sponge as that seems to be cheating...
Support: Replaces Sand blocks (can't be placed anywhere else?) with a darker shade that doesn't fall. Some kind of limit on the number out at a time would be interesting.
A Spray Can for a limited number of colours on some of the blocks would be helpful.
