Part of the game preferably - maybe begla can offer some pointers on where to start in the code, I'm honestly not totally sure myself
We have the little spinning cube as a UI element of sorts, but that's drawn via OpenGL. I'm not sure if that's the path we should take in general or use more general UI components drawn on top of everything.
The first thing of use would probably be a little 10-slot bar for tools
If you feel overwhelmed by the code, that's ok too! Start by finding something small to tweak rather than creating something totally new from scratch. Experiment some, even if it isn't meant for permanent introduction into the codebase (just spin off a branch from your develop if you want to do some throwaway tinkering like that)
For instance, a smaller UI elements of sorts, the Console currently just writes text straight into the left bottom corner, and can't do multi-line commands. Maybe sort out a way to enter multi-line commands (maybe by making CTRL-Enter spawn a newline? Arrow keys able to move around the panel? Switch it to use a text area of some sort instead?). Could also go with a slight darkening of the background of the bottom of the screen when the console is up, sort of like a translucent panel. Could re-introduce the parsing of very simple non-Groovy commands like "respawn" in some fashion maybe.