Wow, that site's a goldmine. Thanks for the links :)

And I was actually looking at ways to use the city/castle split during the MII phase- I mean, they wouldn't exactly be "castles" because of the timeframe, but the idea that you would have settlements that are rich but vulnerable and others that are centers of military activity would definitely add to the game. It should be pretty simple to implement code-wise, you would just have Castle (or, let's say Redoubt) build trees and City build trees that would be mutually exclusive. Design-wise, however, it would add a lot of work to balancing the maps and to designing new buildings. I guess at this point it's a "I'll cross that bridge when I come to it" situation, I'm prioritizing map design and factions for now.

Also, just to see how this idea goes over- having read around a bit more, I'm starting to like Asai Nagamasa's approach to handling ships, namely getting rid of them. To compensate, I'd be increasing the base trade income of territories substantially, increase the rate that Trader buildings increase that income, and have Ports generate their own (possibly fixed) income. I'd also be adding lots of land bridges. Honestly, the idea is appealing to me because it removes the opportunity for the player to exploit sea trade and keeps the AI focused on its immediate neighbors, as well as better preserving the Old School Total War chess-type feel.