I don't mean automatically after you discover the tech. That would be way too OP.
How about a system where, every few turns, your starbases would accrue enough "xp" to upgrade on their own. When they do, it also increases the maintenance a bit. When they level up, you pick the module you want.
There can be modifiers added to how fast a starbase grows, such as various techs in the engineering tree or events. Races like the Iconians would get a free module as usual (and that pragmatic perk would grant one as well). The Iconians with that perk, for example, would start with a level 3 starbase. Maybe race traits could modify starbase growth as well.
I can also see influence affecting growth rate. In your territory on a tile that is 100% yours, maybe the growth rate is doubled. In deep space, maybe it's slower, or in another faction's territory there is a malus to growth.
Every level it would take a little longer to level up, regardless. By end game, most of your starbases should be level 30 plus and very expensive, but you should be able to afford them with better money tech.
I like this mainly because it fixes the constructor spam issue. You only have to build one constructor per starbase which is very manageable and it still makes the constructor relevant. Increased maintenance costs make sense for larger starbases and is more realistic, and also balances the fact you no longer have to physically invest in it with another constructor. You ARE still investing, albeit passively, with money and more importantly, TIME. There should be an option for lowering starbase maintenance to have more control over how fast they grow/how much money you need to save. If push comes to shove, you should be able to decommission a starbase and get some of the money back, from scrapped materials and such.
I love the game but as many have noted, constructor spam can really suck, on the bigger maps especially. This would all but eliminate it.
As far as realism, I sort of see it as the maintenance you pay for starbases is also paying the leader of that starbase to invest in his domain in order to benefit you..,and like an investment, those benefits come back over time.
To summarize:
1. You don't have to spam constructors anymore. Yay!
2. Getting "high level" starbases is still rewarding, because it takes time (and money) to get them. They are also more valuable, because you CAN'T just build a 10x constructor and get it back. I think it would make sense, like interceptors, for starbases to get their weapons and scanner modules for free, at increased maintenance. This would make them harder to take down, and taking down a starbase becomes an even riskier proposition, which makes sense given that this system makes them more valuable, and prevents the player from abusing AI who don't upgrade their weapons.
To balance, maybe constructors can be twice as expensive to build.
Also, it might make tall play more viable. If you invest in starbases earlier, you will beat out civs who opt to expand early, at least in your core system. Your bases will get more growth, and investing in them early will net in the most benefits in the long run. Cost/benefit anaylsis is usually a good thing for 4X game.
Do you guys see this as being too powerful? Too weak? Too radical?