Hopefully I'm not spamming the forum with two Neptune's Pride posts. I've been playing around with these ideas for some time and I actually remembered how to log in this time. (hah!)
What if Terraforming, instead of just a static boost across all stars and systems (including old starting systems and freshly conquered ones), isn't?
What if Terraforming instead, affects "Terraforming Facilities" or "Terraformers", which builds "Resource" in a similar way that Industry builds ships? Say, it costs the same as Science facilities and gives (+0.5 + 0.1x) resources per day, where x is the terraforming science level of the empire? These resources would, like normal, reduce costs of other facilities.
The Capital star system could start with Terraformer level 1, and slowly build up a large buffer of terraformed resources which would over time make the capital star system cheaper and cheaper to develop. This would also make "old" systems more valuable than "new" systems.
Additionally, you can have "Terraformed Resources" become pillagable (for cash) or destroyable (to deny resources) or even just plain destroyed on capture. This would result in more expensive rebuilding process of a subjugated region. Even if the terraforming facilities themselves were maintained like industry/science instead of destroyed like economy, it's still a big hit. It'll make raiding more effective and conquests, less.
Also, destruction doesn't need to be complete. Maybe 50% of terraformed resources are destroyed per capture.
Other thoughts: I hope this would make turtling the mid game a bit more effective, as well as increase the relative wealth of an empire's old systems versus new systems. I hope adding a new facility won't affect the design too much (the 3 facilities really have a nice look to it once I tried to envision where a fourth would fit in) or the balance too much (a capital never captured could end up with insane amounts of terraformed resources, for huge cost reductions).
I'd really like to discuss.