Since Diplomacy will be the last microexpansion, how about letting your modders fill in the gap while you work on the next project (abeit an expansion or Sins-2)
1. Design Contest
Hold a competition for the next two races. Of course you guys would have to handle the balancing issues. But at least allow modders to submit their ship designs. Designs would have to contain elements to support their function and similiar design themes to suggest one race. You can set the rules.
2. Mod packs
Embrance the promotion of several of the more popular mods. Not just a section in the forums, but part of the website where the mod could be offically promoted and voted upon by users.
3. Map of the week.
Post scenerio driven maps promoting single or coop gameplay. This could fill in the need for a full campaign (at this time) and allow for active forum debates about the map and how good or bad it was. Maybe have a leaderboard of those that had the best/fastest endgame solution.
4. Promote Clan Elimination boards
Have a section of the webiste dedicated to semi-sponsored clan matches. You could have monthy matches where clans submit to prearranged battles and post an elimination board on the website. Clan leaders can submit recorded games as evidence of their victory.
As companies embrace DLC as a revenue sources, modders are having more difficulty finding titles that support their craft.
These could all be combined at some point as a user expansion pack. Egosoft is doing this with their X-Superbox.