This is what I found on the economy/geography in a small example focused on the sole island of Shikoku. If this idea pleases you I will then move to Kyushu and slowly I'll build the background on all Japan.
Naturally, source lacking it's in part taken from Google Earth, in part from other games and in other part taken from my cylinder in an attempt to rebalance on one side and to give more accuracy in predicted results on the other which aim for historicity of course.
Naturally since I had to pause Sengoku to take the data the laptop switched off and I lost my first document which was much more elaborated.
I think it's possible to apply these concepts to any historical period but naturally, my research was more effective in Sengoku also because of the many games on this period (even Takeda III) lol.
The research is split on Kunis and it also comprises a few notes where events may trigger very important strategic consequences and it's on those consequences that we may achieve historical results favorable to Chosokabe so that he would become a leading power in Shikoku.
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Iyo (insert clan Kono)
Uwa - Low fertility
Ikata - Low fertility
Ukena - very low fertility - Iron Mine
Iyo - Medium fertility - Capital (trade bonus) - Sea trade (trade bonus + attitude bonus/malus + events with overseas clan in West Honshu and East/South Kyushu)
Kuwamura - Low fertility - Fishermen (income bonus)
Uma - very low fertility - Iron Mine
Notes: possible interesting situation in which the mines of Ukena and Uma generate negative attitude from all 3 neighboring clans to make them seek conquest. This can be achieved by events (insults, hunting accident, marriage gone awry between courtiers, rumors of plots, any possible casus belli triggering from these 2 provinces with the owners of ALL neighboring provinces more strongly on the Kori of Tosa which borders the mountains to those mines).
The insertion of the clan may be an independent faction or subdued vassal to a bigger clan. Personally, I'd like more clans to start smaller.
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Tosa (insert Clan Chosokabe)
Hata - Medium fertility
Tosa - Low fertility - Capital (trade bonus) - Wood manufactory - Sea trade (trade bonus + attitude bonus/malus + events with overseas clan in West/South Honshu and East/South Kyushu)
Nagaoka - Medium fertility
Notes: Capitals had markets and trade routes but when Capitals were by the sea those trade routes were much more efficient. We can use again events to simulate extra trade income (occasionally) and to build rivalry on trade lanes between neighboring clans. Chosokabe would be trading with Kyushu via Osumi/Satsuma/Bungo and with Honshu via Kii/Bizen/Aki.
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Awa (insert clan Miyoshi)
Miyoshi - Low fertility - Capital (trade bonus)
Itano - Medium fertility
Kaifu - Low fertility
Katsuura - Low fertility - sea trade (trade bonus)
Notes: Miyoshi would be affected by the mines event as well. If we can make hatred between Kono and Chosokabe for instance Miyoshi could be involved in a subsequent AI plot. Bad attitude favors war because of the lower honor penalty in case of DoW. Slight events with negative attitude towards the owner of Awaji due to piracy attacks in the straits.
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Sanuki (Insert clan Sogo)
Toyota - Medium fertility
Utari - Medium fertility
Kagawa - High fertility
Ouchi - Medium fertility - Capital (trade bonus) - Sea trade (trade bonus + attitude bonus/malus + events with overseas clan in West/South Honshu and East/South Kyushu)
Notes: This Kuni should theoretically belong to clan Sogo which should be affected slightly by the events triggering from the central iron mine but heavily by the Pirate events coming from the Kuni of Awaji (a Daimyo being behind pirate attacks would give a perfect casus belli).
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Awaji
Mihara - Low fertility
Awaii - Low fertility - Capital (trade bonus) - Pirates (sea trade bonus + negative attitude with bordering clans)
Tsuna - Low fertility
Notes: interesting political development if the negative modifiers trigger attrition with Awa/Sanuki Kunis.
It's just an initial notedown of what I have in mind. Could be more complex but I'd rather not have the player fight with the screen events and pauses too often. Note the abstract value given to fertility. This number needs to be addressed with tests according to the various economic conditions and upgrades (i.e. upkeep, upgrades, etc.).