In Master of Orion 2, in end-game, when you've obtained most or all of technologies, colonizing a new planet is a pain in the butt. Why? Because you need to queue up buildings that you want to be built. Which buildings? It don't matter, because they all help your planet out (Automated factories usually go first because it speeds up production of other buildings). So in the end-game, you don't really care about which buildings to build, just that you want all of them built. This kinda defeat the purpose of having unique buildings. Unique buildings aren't important when you just want "All of the above" approach.
I wanted to have "unique" buildings/areas on a planet where you just can't have "All of the above", and took my inspiration from MoO 3's regions. While overall, MoO 3 may be a disappointment, but I've found many elements inside it that I really like, and this is one. With regions, you can have different races inside one region, and each region have their own improvements, limiting which buildings you can build on a planet.
However, I'm now at the point where I need to rework the UI and stuff for planets to work under the new system, I realized that I may be looking at it wrong. Having unique buildings across specialized regions across planets across the galaxy can easily defeat the whole purpose by introducing tedious micromanagement with little rewards, where you just don't care about that one little building that boosts farming by 10% in an obscure corner of the galaxy. It also makes each unique building lose their significance. It'll devolve into "I need more food, let's build an farming region on the next available region and build all farming region buildings there!"
I spent some time thinking about how to avoid this and reading people's suggestions about my game (particularly the one over at rpgcodex), and I think I finally stumbled on an elegant solution that will help me achieve what I originally wanted. So here's how I'm doing this:
Regions will be replaced with "SectorObjects". You may be thinking, wait, isn't a planet a SectorObject as well? The idea is that each SectorObject can have a subset of SectorObjects (aka regions). This means that you can be as detailed as you want while modding (Having cities in regions in a country in a planet for example), and the game will handle it automatically.
If a SectorObject contains a subset of SectorObjects, then it derives its population and economy from the subsets and adds them up for the UI to display statistics. Only the bottommost SectorObjects can be controlled by an empire, and as a result, they're the only ones with projects and sliders. SectorObjects can be changed to different SectorObjects as a result of projects (for example, changing from Farming to Mining region, or terraforming from Desert to Terran planet). Sliders are defined in data files. If only one slider is defined for a SectorObject, then it's hidden and always at 100% (this allows you to mimic Moo 3's regions).
What does all of this mean to you as a player? It means that you can share planets with other empires! Owning a SectorObject requires that certain requirements are met, such as sacrificing a colony ship, creating a project to colonize one, or having migration be able to migrate there without owning it first. Once owned, your people will automatically migrate there, reducing the need of manually moving around your people.
Now, what about ground combat? How will they be handled? I think I have another brilliant idea. Since each subset SectorObject has a parent SectorObject, you can choose to either invade the whole parent SectorObject, or invade each subset individually. So if you're invading a planet that is 50% owned by an ally, you can choose to invade only the enemy regions, leaving the allied ones alone. However, if none are friends of yours, you can invade the whole planet. The ground combat will be handled like MoO 1. Also, if you feel that you're outmatched, you can choose to invade only one region and establish that as a foothold. Same goes for planet bombardment (selecting all of the planet, or just a few regions to bomb)
How does this help me achieve "Unique buildings"? Another change is that the SectorObjects will have "limited space" for development. This means that if a region have only 20 "Space", and you build something that takes up 10 spaces, it'll force you to choose carefully on what you want to build. Also, with sliders back in, you can leave a region "Unspecialized" which don't give any bonuses or penalties for producing something, or change it over to specialized. For example, if you change from Unspecialized to Farming, then the farming output is doubled, while mining, industry, and commerce are all halved. Then you can build few certain buildings (some may be restricted to certain types of regions, such as Automated Mining for Mining regions only) to further enhance the bonuses or to alleviate the penalties. Each region can have their own special bonuses/penalties such as fertile grounds, radioactive deposits, etc. If you change to a different SectorObject, any improvements that don't meet its criterias will be automatically scrapped.
All of this can be managed from one screen that for now I'll dub "Economy Management", you will be able to filter out planets or regions, apply broad changes (such as increasing food output to 50% across all farming regions) or even individually manage each region and their project. This should help in reducing the tedium in managing planets, while allowing hundreds of planets under your control!