I was thinking of giving each country a “logistics” score.
It represents the practical resources you have available to do fun stuff like invading other countries and sending ships around. Having troops outside your core would drain your logistics score until you had nothing left, while getting your defensiveness high could actually increase the logistics burden on people attacking you. In theory, you could keep all attackers out excepting your border provinces with a supreme defensive strategy, although this would wreck your own logistics score. Meanwhile, high logistics scores could actually improve the effectiveness of your soldiers in battle.
Waters defined as coastal wouldn’t have a cost, but otherwise the AI (and the player) would be forced to return fleets once they were runnign low. This also would give a neat new mechanic to balancing invasion - you can’t just go racing across the enemy country at will, but must pick and choose where to invade. Since hilly and mountainous provinces have higher
Friendly/allied provinces would have a very low logistics cost. I further think you should be able to use a decision or temporary structure to create forward supply bases (yes, it sounds modern, but the idea isn’t) as you conquer. The human player would probably need a “logistics” map like the relational map.
The upside to me is that this appears as though it would require fewer processor cycles (it’s one score per country, and doesn’t matter in peace much), and be more useful for AI decision making. it would not be that hard to, though the AI will still be a little predictable. I’d probably develop a few AI options for high-logistics/aggressive, mid, and low countries, with increasingly daring moves for the AI at higher levels.
Low risk AI would tend to stick near the border and invade on the most congenial ground. High-risk AI might try to dash troops in deeper and attack tastier targets with more loot, and which hurt you more.
However, I tend to favor some things which go with it - like breaking provinces up a bit, into somewhat smaller groups which can individually be fortified (so that not every inch of Europe is a fortified bastion). This would open up invasion routes and give players options as to what they want to fortify, etc. The developers can also strategically choose what to put forts on so as to limit all countries based on reasonable historical outcomes.