I might be wrong, but I think Fatbag may be reluctant to contribute to the game engine reimplementation as he doesn't have much game development experience - despite that though he is smashing at reverse engineering, x86 analysis and algorithms (and writing clean, professional code, because mine is an absolute mess!), which have all benefited us greatly.
Buy/Build is very "gamedev" but there will need to be a lot of backend changes to allow dynamic edits to the objects while the game is running. TSO does not pause when in buy/build! Moving objects creates a temporary "ghost" object that is only displayed locally and has no real function, until the object is placed, at which time before placement you need to verify that the object's Occupied flag is not set. That might make it quite hard for anyone else to contribute to, as the SimAntics engine will need to be modified.
The other mystery element of improvement is the HIT system, which just barely works right now (90% of sounds - ones with special code break down). That might be easier to start contributing to - I've tried to keep that section of code clean, despite the huge amebo-style interpreter class & table.
Also, he has previously stated that he's busy with other things, so I think he doesn't want to become committed to PD.