i think that having more builds / ability to mod them would be really good like I said, especially for MP games.
However Im not sure that the AI could "use" these new buildings very well or not. Example the AI would build a specific new structure to recruit units from it? Im not exactly sure that the AI could handle the situation, but who knows.
Also keep in mind, that NTjedi talked about mods. If IW would add this modding feature [adding buildings and editing them]...well I cannot see anything bad about it, in fact the modders would be happy with it. If some people wouldnt like to see more buildings in mods...well they wouldnt be forced to use the mods afterall, am I correct? Theres the vanilla game for everyone.
