Looks great. I have some suggestions.
  1. Provide access to UI interfaces: IBaseUI, IGameUI, IGameUIFuncs, IGameConsole, IEngineVGui, etc. (an ability to manipulate the UI without accessing the engine / GameUI).
  2. Add missing interfaces inside server dll: NEW_DLL_FUNCTIONS / SV_SAVEGAMECOMMENT.
  3. Make a new generic mathlib that can suit all solution projects. You mentioned adding vgui2 support to client.dll, which is great. But putting it all in there is a pain in the ass. For example vgui2_controls.lib uses mathlib.h from Source SDK which also includes its own vector.h header file, for client and server we already have vector.h and util_vector.h and they will all hate each other. This could also be related to the tier0's platform.h, which will conflict with Platform.h from vanilla SDK. Creating common header files will not confuse people and take the project one step further.
I have many more suggestions (for example hacking the engine data like server_t / server_static_t etc), but I need to see what will be added next, so I will not suggest anything that could potentially go beyond the scope of the project. Best of luck with it.