A topic in blender helped remind me of features I'd like in Terragen for light sources.
Control over the light frequency (aside from just colour), similar to Blenders Blackbody node, and I'd love proportional outputs (maybe wrong terminology).
I wanted to demonstrate light source colors recently for nursery environments, but realized doing it in TG and being as accurate in simulation wasn't possible so I had to use blender with some cubes, as I don't know how to do crap in Blender. I found this which was super helpful for simulating real-world lighting, and this would be amazing in TG: https://blender.stackexchange.com/questions/180507/simulating-a-40w-light-bulb
It also seems lumens are effected by size, which is interesting. Don't know much of what's happening there, but makes sense (bigger filament/light source = more lumens)