I voted debugger. IMO the very useful "find references" should be joined with debugger as the debugger needs to dissect a project anyway. I think its worth considering how useful a nice, user friendly, debugger could be for learning C2.
My second choice would be for modularity. This would be awesome for teams. Abstracting complex events as simple modules matches C2's philosophy for simplifying the process.
Third, I think this pole would be much more helpful (statistically) if we could rank our choices rather than selecting our most preferred.
Fourth, all the options sound great and make for a hard choice. C2 is awesome!