Tungsten. (TNGS) It would glow in contact with electricity. Now, before you bash on it, it could be used as an indicator for electric charges. Also, think about the "usefulness" of other elements, like soap. TNGS would be very bright, and would make light-making easier and more efficient.
I've tried the plasma method numerous times, but it ends up with a grainy lighting effect. And it would be nice to have an element that doesn't require decoration to glow. Tungsten wouldn't melt the metal either, something that noble gas does. Plus, it would be extremely bright.
Hm... There seems to be a cglow variable used in render_parts, it uses some program fireGlow to create stuff, but IDK what cglow needs as a value(glow size? glow colour?)...Anyways, it can be done, there just has to be some cglow set when you instantiate your element. >edited, jacksonmj cleared it out