code1949:
As the search function is currently non-existent (and as some people NEVER use it) I have decided to provide a guide on what not to suggest and why.
Movable Solids: they are very difficult to program.
Layers/3D: Way too much lag.
Sound waves: Pressure = Sound waves.
Voltage: Simon is already interested in voltage, there is no need to suggest it to him.
Animals: Completely Useless. Will cause major lag.
Extra Stickmen: two is more than enough very few uploads even have stickmen.
Super Stickmen: there is no use for them.
AI Stickmen: AI is difficult to code and AI-controlled units have little use in powder toy.
Vehicles: Hard to code, almost useless.
Other Stickmen Related Things: Completely useless.
Nuke: Bomb is enough.
Lasers: Just use heated photons
Mercury: Use water or liquid rubidium.
Corrosive liquids: Use acid.
Perpetual Heater/Coolers: Use GOL and HWSC.
Wall that Blocks Air Only: Can be achieved through other means.
Hydrogen: Planned by Simon for the future.
Substances that change volume with heat: Deuterium and Yeast are enough!
Magnets: Too much work required to implement.
Biological Substances: They are completely useless.
Pixel Thick Wall: Will cause TPT to run at 1 FPY (Framerate Per Year) if implemented.
Also here's somethings to consider:
Diamond is inert and invincible and nothing will destroy it.
Make sure it is significantly different from pre-existing elements.
Make sure there are many uses!
This is a suggestions forum this is not a place to spam or start flame wars.
Remember, if you want an element that badly, code it in yourself.