Well for a moment I entertained the idea of a 'final expansion' that added content as well as some kind of platform for custom content and workshop support with that. So, for instance...
Bonus Ship Workshop: Create a mark I ship of any specifications. Specify its list of immunities, abilities, and so on. Is this bonus ship available to the AI? If not, create the AI's modified version. Which statistics scale up with mark? Do higher marks introduce better abilities? Is this bonus ship a fleet ship or a starship? Custom Bonus Ships, if enabled, will disable achievements due to the possibility of creating massively powerful 197-cap fortress-like ships.
Galactic Jumper: Create the planets, names of planets, the layout (including base layouts) of a galaxy... and place any ships you want in each planet to create an entire campaign! Using the high technology humanity has acquired, they're bound to find that layout somewhere in the massive universe. If you so desire, you can even tie waves or other things to various events and put any timer/ships you desire on the wave. Or, of course, you can make it AIP/Difficulty dependent. This editor also includes scenario editing for Ancient Shadows. Any edited or custom scenarios can be easily abused, and will not count for achievements.
Tweaking Station: Create bits and bobs for regular campaign use. If these bits and bobs are allowed in a campaign, achievements are disabled. Bits and bobs may include things such as hacking or knowledge gathering structures, 'Data' structures, command stations, base structures that the human player can build by default, turrets, knowledge unlocks, modules, anything not covered by the 'bonus ship' category. Present ships cannot be removed or edited, but can be used as a 'base' for new things.
That expansion would not only like, blow everybody away in terms of regular content, and go out with a bang, but then anybody can just add ships and such to the game. If there's no desire to make custom art assets on the player's part, they can just use existing assets instead probably. I mean they're all already there and uncompressed.