After the ancient alien gold synthesis lab was added I realised that most advanced versions of resource producing buildings now had an ancient alien version, making it easier for advanced players to get the tons of resources that they needed. But, after a certain point, your demand for plastic rises a lot because you need it for the triantanium refinery and since it isn't available on the stargate you will need to make a lot of advanced plastic factories that will fill up most of your space.
So, perhaps in the next update an ancient alien plastic factory could be added to help more advanced players get more plastic faster.
This version of the plastic factory could still use robots besides all the other materials needed (such as oil, uranium or alien artifacts).
It could also be as if not more expensive than the alien enritchment facility to not make it so overpowered.
Or simply make the plastic resource available on the stargate.