What about adding a generic trading post as a common building, and a Kontor as a UB for a certain civ?
We can also add warehouses and trading offices to follow the idea.
For which civ, the Germans?
(Off topic, I do think a modmod that adds a third Unique to each civ would be a good idea, it's an opportunity to bring back all that kicked out naval and air units and difersify the civs a bit)
How about letting us use up Great Admirals to create "channels". *snip*
be a good idea. But I do think the producers would have implemented it if they saw a way to do it without generating too many problems. For example, what do you do with melee ships and land units? Is it codeable
to have a land unit block the thing? Do you need to code an entirely new terrain that both types of units can walk on it. I'm all for it if it's doable though... (and I'm no modder in that way so I really can't tell you, but my gut says no...)
I mentioned this in an earlier thread: establish buildings that are basically Military training academies to give specific bonuses to units garrisoned there long enough to be "trained" in the specialty.
If you made this too easy to earn, then you'd end up with armies of super-units with multiple specialties, all achieved in too short a time. The idea is to allow specialist units to be groomed for specialized tasks, not to give generic army-wide bonuses. This also allows peace-loving empires to be able to build some strong specialized units in the absence of actual combat experience.
I'm not sure, there's always the balancing act of usefulness for buildings. Those have the potential to be overpowered (but you showed how that can be avoided), but they also have the potential to be very niche. Contradictory, they may also provide a way against bad luck in terrain rolling, lessening the luck factor. One place where the civ series excels in my mind is generalization. Not everything needs to be spelled out in detail. Long lists of possible buildings to build may look bad or discouraging if you only ever build one or two. I don't know, maybe they are good buildings, or maybe that can be a (fairly often happening) opportunity where you need to chose between a mountain or jungle specialist for you Military Academy. Or between a Marine and an Airforce Officer
Another building for the modern era is a Metro, (ot Public Transportation System), giving commerce or hammers per people.