Now, I know this element has been suggested before, but I personally suggest that platinum be added to the game. In the Powder Toy, platinum could be very useful because of its real-world properties.

Realistic platinum!
In real life, platinum is one of the "noble metals" (like gold), meaning it is relatively inert, but it makes for an excellent catalyst. I forsee that platinum could be used in TPT to speed up chemical changes around it (although not to itself, catalysts don't work that way) such as GLOW and WATR turning to DEUT. I can't list all the possible catalysis reactions that platinum could be used for, but I'm sure it would be a worthy addition to TPT.

Realistic deuterium!

Now, onto deuterium oxide. I know we all make amazing bombs with it, but it is wildly unrealistic. I'm sure there were reasons behind making it that way, such as making it behave like water not being fun, but maybe we could tweak its properties to reflect its real world uses? Deuterium oxide is used IRL in nuclear reactors to slow neutrons, not to generate more. Just so everybody knows what I'm talking about, deuterium is an isotope of hydrogen, therefore its chemistry is barely affected. My point is that deuterium oxide should behave almost identically to water (aside from being denser, of course), and should be able to be electrolyzed just as water is. Because deuterium will be one of the products rather than normal hydrogen, we should still have a distinct use for it, right? I propose that deuterium gas should be easier to use in nuclear fusion (and release more energy) than straight hydrogen.

A scientific explanation as to why deuterium should be easier to use in nuclear fusion

First of all, nuclear chemistry will tell you that the electrostatic repulsion generated by protons needs to be balanced by the presence of neutrons. With the right amount of neutrons in the nucleus, the strong nuclear force holds the atom together despite the protons wanting to fly apart. This means that if you have NOBL, which I can only assume is helium, you need neutrons in there if you don't want your atom to fly apart. So using deuterium, which does have a neutron, unlike hydrogen, you can create stable helium in the nuclear fusion process.

I'm not sure if having unstable NOBL is right for TPT when it's the product of normal hydrogen (despite that being more realistic), but we should at least give deuterium a marked advantage over hydrogen in nuclear fusion, making it a clear better choice.

Before anyone states this, I'm well aware that TPT is not meant to be completely realistic, but it would still be nice to make some things more realistic, and besides, there needs to be some realism for it to make any sense, right?