| So whatever is holding starbases in position in the game now is somewhat a mystery? |
The Starbase heavy mass would require powerful engines and possibly lots of fuel to move it. If you want to use a large structure in space and do not necessarily need to move it, you leave out the engines (and fuel) and you have a starbase.
If you add engines then you would get essentially a very big spaceship. I can however imagine that you would want to build a structure in space that is stationary most of the time, but has engines or can be towed. This could still be considered a starbase i guess.
In my opinion I would like to see powerful starbases that do not necessarily need fleets for defense. To be able to move them as well would make them a bit to powerful then and make starbase placement less critical, which is less fun I think.
I like the distinction between movable ships and immobile starbases as it is. Even though in sci-fi, anything is possible, a game has to have distinctive elements and concepts or else you run the risk of game functions getting 'blurry' which Galciv already suffers from sometimes. Consider for instance the abilities, super-abilities have now been introduced because a lot of the ability effects are hardly noticable in the complex game world of Galciv.