The main things that come to mind for helping with development itself are UI stuff (refinement of the existing, replacement of the existing, new bits entirely) and balance analysis/changes. Both of those are pretty much wide open in the external code. But neither of those is to the point where you wouldn't be subject to it all going poof fairly soon anyway.
The AI will also be exposed more in external code later, but not much of it is there yet.
There's also turret patterns, but that's largely based on balance and UI considerations.
So if you're bored I'd suggest adding a new map type based on some pattern or algorithm you find interesting. You could start with changing an existing one, and then move to creating a new one from scratch (probably by copying one and replacing the innards).
Thanks for your interest