I feel that the best thing that could happen with this is someone made a really, really basic, version which was open source and well commented. From that, I'm sure there would be more people willing to actively take up the mantle
What minimal feature set would you want in that version?
For the minimum viable product, I'd say:
- able to move ships full of marines from planet to planet
- different planets would be filled with different enemies and battle would commence when a ship attacks it (in a text based log, no need for graphics).
- enemies would spread to other planets per turn(s).
- periodic or action based events with some choice (going to planet Balthazar might give you the choice to enter a tomb and potentially get loot, but you might lose soldiers etc.)
- marines could be equipped with different gear which would affect their stats (this could be pretty broad to start with - the equipment could be set per ship rather than individually for instance).
If you had that, along with the things to go with it (an inventory for each ship would be needed for instance) then it'd be pretty easy for others to build on - events could be created, enemy races could be built, battles could become more nuanced etc.
The key would be to have it super well commented, and with as little spaghetti code as possible. I'd also suggest it be done in a more global language than Gamemaker - python, C# or similar, as gamemaker can be a bit restrictive and isn't really suitable for githubbing etc.