I would add a tidbit I learned from reading the coder's responses:
The Freespace SCP engine is way too interrelated. You can't recklessly modify any single part of the code since other parts could rely on the same data or use that specific implementation you're about to overwrite. (It's like house where, when you paint the living room, suddenly the carpet changes in the bathroom... and doing anything to the plumbing in the basement runs the risk of setting the kitchen on fire).
To get anywhere the engine will have to be modularized. Granted this could be done with any single "part" of the engine (like how it was done with the AI) but when you get right down to it, you'll be faced with gargantuan monolith that casts shadows everywhere: The Rendering Engine.
So if you really want to see it another way, the projects aim is to separate this monster from the rest of the engine. (It's prime objective is not to "improve" the beast, but to pull it apart and turn it into a manageable dragon instead an all infesting hydra).