We've added LUA scripting language support to the Source engine (this is done expertly in Garry's Mod and we've learned a lot by looking at it and talking to him). We are then treating the game more as a library or API, with most of NS-specific code implemented in this scripting language. This includes things like setting up the tech tree (including costs, research times, dependencies), spawning (wave spawn, spawning at structures, etc.), the resource model and even how all the structures work.
Hopefully that will enable the game to be coded a lot faster :D.
This also means maps can pre-place structures, allowing for more scenario-type maps.