I suggest gasses's original brownian motion (that jiggling randomness in the position of the particles) to be varaible depending on temperature. It wont break saves, but will be a cool addition.
Lower temperature - more still particles.
Higher temperature - more intense brownian motion.
Nice... this could create a lot of new elements, like solid and liquid hydrogen and other gasses.
This *WILL* break saves. just saying
Yeah, also, dont forget that the positional noise should change, not the ability to move. It should not become solid when near 0 Kelvin, otherwise it will break saves, as moonheart08 said.