I would like to see a shipyard mission to ship resources from one planet to another to help building stuff. It could be called something like "Resource transport" and on arrival at its destination it would then contribute like 80% of its resource cost to the social production of the planet (the remaining 20% were needed to build and fuel the transport ships). The same could go for a second mission type to provide military production.
That would make empires possible where you only have a few highly developed planets and a lot of others that only provide resources to them. To make that a viable path you would have to introduce more expensive high-level improvements though, otherwise the planets could max out too fast with the current set of improvements if they are supplied by a bunch of resource planets.
The only problem with that is right now that the mission mechanism would have to be adjusted so that the entry of a specific target is possible.
Alternatively there could also be a new ship type "Resource transport" that is built like a normal ship in the shipyard, but instead of loading colonists or legions it can load social or military resources and unload those at a target planet. That would have the advantage that the mission mechanic would not have to be changed, but of course you would have to implement the resource load/unload mechanic 