Already in and has been since they were introduced.
I should have indicated simultaneous multiple sponsors... that did not appear to be the case in 3.1...
What do you mean? You want to make a fleet of starports? Why?
for parallelism in the queue and transparent fail-over/fall-back for build orders... features that you really do want in a hostile environment (real or virtual)
So you want starports to be able to build starbase modules? Then why would you ever build a starbase?
I think that starports should be a derivative class of starbase (they are all space stations of a sort) - so you start with either a 'general' starbase which you can specialize with the economic ring or the culture ring or you can start with a 'pre-specilized' starbase called a starport that you cannot specialize any further... To either kind you can add the 'common' offense/defense modules and/or the 'appropriate' mining module, etc...
As to why have starports and starbases - it's about the specialization... a starbase with a 'hanger' ring and/or a heavy weapons module becomes a battlestation, etc... on the back end it will actually make for better code maintenance, since both will derive from the same class... as for playability it will actually make it easier, again since they should come from a common class, playable object capabilities management will be simpler.
[I do hope they migrate the 'anchor' -> 'unanchor' -> 'move' -> 'reanchor' functionality to the 'general' starbase class - it adds a tactical wrinkle that should/could be interesting]