I've always felt that the building system in CK2 was just too shallow, and that whilst Flogi's is a good upgrade, I've decided to have a crack at this sort of thing myself. I'm going to make my own building mod, in the vein of the one that could be found in Lux Invicta, but more hierarchical and with the aim of making provinces more unique and giving more thought to the selection of provinces beyond how many holding slots they might have or whether or not they're a coastal province.
So, some concepts for what'll be Caek's Building Mod. I plan to make this thing publicly available on the Workshop if I do well enough, but for now, some basic concepts.
1. Buildings in a province should be an interrelated weave of buildings with dependencies on other buildings in the province - you shouldn't be able to just build straight towards a doom fortress of a castle without any supporting infrastructure or craftsmen to do it with.
2. Province terrain types should have a meaningful impact on how that province develops. The resources and construction possibilities of a mountainous region such as the Alps are radically different than those you can find in Jerusalem, or in the middle of France. Different terrain types should have access to different building chains, using the game's inbuilt system of terrain requirements to determine what building sets a province should have.
3. Troop composition should be influenced by the above to give different geographical regions more flavor and to better match the actual realities on the ground. It doesn't make sense for a mountain realm to be rolling out as much heavy cavalry as people in the fields of France with plenty of pasture lands, but it would make sense for them to be
4. Buildings should be both a meaningful investment of resources with a meaningful impact on the game at large - even if they are just a drop in the bucket, they should still have an impact on the game, even a small one.
5. It should represent well the structure and realities of the Medieval world.
6. It should offer some impact on events and other possible courses of action.
7. Cities, Castles and Temples should have meaningful differences both in how they are developed and how they are
8. Education should be of consideration.
Fortunately, I'm actually rather well versed with modding CK, and all of the above should be possible. Basically, the building list will be divided into three major categories: militarization (containing things to do with levies, retinues and fortifications), economic development (containing things to do with infrastructure, resource gathering and craftsmanship, that sort of thing) and cultural development (containing things to do with entertainment, education and some administrative functions), with Castles, Cities and Temples specializing in one area. I'm thinking about having a special "building" for each province that can't be upgraded by construction, but by event, which would act as a gateway for things like education level and other conditional things, but we'll see
All in all though, I think this would be rather fun to build and play!