My take on buildings: when you are small they are really darn expensive and it takes forever to try to get anywhere with them. But when you have grown a bit you rake in so much cash that mass building becomes stupidly trivial. But the problem is also that building buildings is one of the few ways you have to interact with your realm's lands and development (a few largely set-and-forget councilor tasks aside). So less building also means less realm development and management and . . . I don't think that's something the game needs less of.
So building income alone starts off really slow and then spirals. And when combined with vassal income (which can often be easier to scale, especially if you get yourself into leading an administrative realm), this really goes off kilt. Some of thoughts are (the first two I don't really care about nearly as much as the others):
1) Barony level building upgrades (e.g. castle level) should be maybe more expensive
2) Lower levels of sub-barony buildings should probably either provide a bit more gold or be cheaper to make it so that you don't have to wait several decades to build a small addition
3) Higher levels of barony and sub-barony level buildings should have tradeoffs in the forms of cost financial or otherwise to reflect ongoing maintenance and infrastructure costs
4) Military buildings that exist to boost MAA should also raise the costs of troops deployed there depending on level - marginally when raised, but also substantially when
not raised (to reflect that your buildings are making better troops by presumably making them more professional, which means more training around the year) - something like 10%/15% per level when raised but like 20% to 50% per vel when not raised. By late game, a fully boosted MAA should cost decently more than they did at game start during war, but substantially more during peace especially to represent that these guys are now basically full-time professional soldiers.
Additionally, bonuses BESIDES the cost increases (which would be applied immediately), should have some sort of ramp-up period before they trigger - either a soft or hard build up, where they climb like 15% every month until you have the full bonus after like 6 months, or just a hard "you need to have been here for at least 6 months to get the bonus" cutoff - there should probably also be ramp downs for the increased unit cost (which I would say should almost certainly be gradual) - this would help prevent shenanigans with stationing and unstationing troops for brief moments to pick up their bonuses and avoid paying maintenance costs over time
5) I would actually love the ability to develop new building slots for meaty gold costs that also come with a chunky maintenance cost that increases with each extra slot you add per building level. This could be in place of or in addition to the existing tech slots. My ideal way would be that each "you can build one more building" tech level actually gives you a modifier to that cost. So let's say by default a barony can build 3 buildings, it'd be something like:
Build Cost: scaling_modifier * base_slot_cost * min((total slots - (3 + tech_modifier)), 0)
Maintenance Cost: base_maintenance * min((total_slots_built - (3 + tech_modifier)), 0) * scaling_modifier
So every building over 3 would impart scaling costs to build and maintain. But if you unlock tech X then it will either discount the maintenance of your 4th slot if it already exists OR make it cheaper to build and maintain that fourth building in the first place if you haven't already.
6) More buildings, even if it's just splitting existing buildings into separate different buildings (and adding slots to reflect this rebalancing around splitting) with portions of the bonus, would be nice as it makes you interact with the realm a bit more instead of something you wait several years to do and then make three clicks. And then you wait several years more. And then make three clicks. And so on. Ideally the alternative would be more substantive realm management and interesting buildings, but even this simple level of interacting with my baronies more than a few times every few years would be nice.
That plus just generally more realm management stuff to do, especially stuff that costs gold would be nice. An easy thing to do would be the patronizing of building projects. This seems ESPECIALLY lacking for the Byzantines, where Emperors undergoing building projects and other such ways to utilize their comparatively massive treasury was a big part of establishing their legitimacy and winning approval of the citizenry of (mostly, though not necessarily only) Constantinople.* I would say that for Administrative Realms this kind of thing should actually impact your legitimacy and be a common part of shoring up new rulers and ESPECIALLY new dynasties.
*And when I say "new buildings" here, I don't mean like buildings that you build for bonuses, this would mostly be stuff that wouldn't materialize on the map or give you bonuses aside from maybe legitimacy/piety/prestige. It's more to represent stuff like emperors (in the ERE, but also just lords basically everywhere too) funding stuff like small hospitals, small temples/monasteries, other general infrastructure and etc.
Something unrelated to buildings is that it feels like more council positions should have monetary costs to represent pay and that both council and court positions should have costs that scale with effectiveness and ruler rank (to some upper bound - a good council should be somewaht expensive, but it shouldn't be bankrupting the ERE either)
Duchy buildings and Wonder buildings should also have maintenance costs (at least the ones that aren't just gold generators) - though those also just need rebalancing in general imho. There are some that are absolutely busted, while others are almost useless and super situational. Also don't get me started on locked wonders meaning that you lose a wonder slot because it's taken up by a mosque that doesn't yet exist, and you're boned because you're either not the right religion or - even worse - not the right flavor of that religion for that province.
It
should be possible to get very rich and have a very rich empire with eyewatering amounts of money - the Byzantines - especially in the eyes of the much poorer and less centralized West - were kind of like this (though decreasingly so), even in their diminished form up until the end of the Komnenoi period.
So if we make too much money, we can't simply say "reduce the money." Because nerfing just makes the game no fun and there's no incentive to do anything. Rather we need to always think opportunity cost. Doing this one thing helps us in one area, but hurts us in another area. Again, Paradox started the game like this with traits being +X but -Y. But with every DLC iteration, it's all more + + + + + and no -.
An Opportunity cost isn't when you have something that gives you +X, -Y. Not all tradeoffs are opportunity costs - the vast majority aren't. An opportunity cost is when the taking of one option crowds out or otherwise makes you lose out on the
gain that you would have made with another option, or in other words: taking +X means you
lose out on the option of taking +Y of some other option.
In other words: An opportunity cost isn't a building that gives you +2 gold, -2 MaA Strength - it's losing out on a +2 MaA building because you went with a +2 gold one instead. This is actually something that CK3 does decently in most areas. The problem is not that there aren't opportunity costs, it's that because of game balancing those opportunity costs aren't very meaningful outside of the early game. Getting a bit more money or a bit more strength isn't very impactful when you're already very rich or very powerful and vastly outscaling everything in the game - and the way the game is currently tuned, the player can often end up doing both after a bit. The game needs more
tradeoffs and
costs to bring the balance back down to where the opportunity costs are meaningful again.
