I agree. A lot of the specialties (swordsmen for Shimazu, archers for Chosokabe) were only included for the sake of diversifying gameplay. I guess in general, I'd like to see diversity between clans and cultures, but I'd like for most of that diversity to come from things like laws, decisions, the ruler's traits and the natural resources of their territory. I.e., the Takeda clan used cavalry extensively because they controlled excellent horse-breeding grounds in Shinano. Realistically speaking, nothing should stop any other clan in control of that area from fielding cavalry that's just as numerous and effective, and for that matter daimyo could have invested considerably in building up a strong horse-producing region in other provinces. I'd like to see specialization like that, where clans specialize in order to make use of their natural advantages, not like the Shogun 2 system where some clans are just better at certain things for no real reason.
I agree. I have already few thought about certain area. F.e. the foot soldiers from Western Chubu (Owari, Mino and Ise) were known as their weakness, but on the other hand, Oda Nobunaga from this region mobilized always more army than his enemy, so we can add special buildings which increase recruiting size.
I don't know much about some other regions, but maybe we can make military diversity with morale and army size (also some ship bonus for western Chugoku and Shikoku provinces where we know the activity of naval factions).
Some local trade goods such as gold mine should be in game too. Magnate Lords Mod comes in my mind, but it is bit complicated for the first stage of this mod.