I think the a good add on would be a new starbase, that can build ships. The first level would only be able to build tiny hull ships very slowly. You can then buy new modules, one parth of modules speeds up contruction, the othere expands the shipyards so it can build larger hulls.

I think that it is probly best if they can only be build in your territory, and cannot get any wepon modules. They may also need to have more module space, but each module would have a large price.

So, what do you think?