The idea that a module is available to all types, but only upgradeable with a certain base type, is probably possible, but pretty confusing to a player who doesn't read up on what the mod does... but I'll test it just to confirm its possible.
Regarding less starbase types = less spam... untrue with GSM. Why? 1) the sensor station requires a planet in its orbit, so its using real estate that could go to an econ base. 2) My current version of GSM (my prototype) increase SB range and SB placement limits by double. The result, is that i had to place these sensor bases quite carefully and in many cases, they stopped me from place 1-2 other bases, such as a easy mining base or econ base. So, it really was an interesting way of forcing harder choices.
But, it would be easier in the files to just add better sensors to the Starbases, so we will see.
Regarding the AI. Good points, but my understanding is that most AI don't even have fog of war, so they are actually wasting resources by building anything sensor related. But very good point, I'll hold off on a "orbital defense base".
Once 1.7 is out for real, and the finalized stats are in, I plan on...
1) Applying a Doubling to SB range, placement limits, and all module effects, while also doubling the cost of a construction module.
2) Adjusting sensors as discussed in one way or another.
3) Taking another look at the MSB, and perhaps giving it some shipyard specific modules that assist construction, so players are more tempted by it.
4) Bring back thrusters with evasion, most likely a %boost and limited to one per ship.
Any other modifications you think are no-brainers?