Let's cover a single area which game revolves around with offering a few glimpses on how it interconnects with other departments.
War
Rebuilding of the lost stacks and mobilization of them to the theater of war is lightning speed if you built up your regions properly (rushing tier 2 roads). Provincial (Domain) capital placement is easy to manipulate by disassembling and then temporary increasing current Stewardship generation in the target region. No longer you have to keep your stacks in the capital to receive 25% Upkeep reduction.
Pillage is more profitable than ever for both Diplomacy (maximizes chance of rebellion to take the region without negotiations) and for the maintance of your stacks. You can afford to go into thousands of negative Gold upkeep temporarily if you carefully LONG Term planned your targets, saved RGDs to create further supporting or surprise stacks right at their territory, stacked resupply decisions that combined with Forage allow you to skip their walled and garrisoned frontier regions and penetrate in the belly of their empire.
Another aspect of the long term War planning is the blistering speed of blocking their Provincial capitals, hitting them everywhere to drain resource bases and National Authority, blocking their ports to put the further strain on the economy, obliterating their Vassals (auto absorb, dont have to negotiate), ensuring Passage Rights and Alliances to attack from the multiple directions and hide your wounded units on the neutral territory for Healing or use them as a buffer to sneak your forces to attack from the unexpected places.
Moreover, the long term planning takes into account the supply bases monitoring. Sometimes its prident to wait until opponent region gets a new POP, so their food supply is drained for the easy blockade. Which regions you want to grab on the way to their National Capital without Pillaging or during the naval invasions. In Empires Roman camps would pull the food extra food into the regions, so you can easily Pillage everything on your way and still be supplied. If you play against human player well garrisoned islands are particularly difficult to assault. You need 2 supply fleets rotating while you besiege them.
The most significant aspect of the planning is an examination of the terrain you have to traverse and fight in (take note on the rivers, and which unit/general mobility and stat bonuses for prevalent terrains will come inhandy), level of fortifications and garrisons (including Military Expertise bonuses), generals pool available to the opponent, availabity of vassals and allies, scouting of the main doom stacks to adjust your army compositions in order to not overspend, therefore having the higher flexibility (faster coverage of territory and ability to regroup and extend/collapse the frontages and compositions) due to owning more stacks.
But there are correlating aspects of planning for the aforementioned war times. The amount of specialized regions you have per province that maximize gold and metal for the upkeep (Metal less important in Empires), growth and health for the manpower (Mercenaries are safe in Empires, since there are no characters who can betray you, no regional penalties for firing them), Local Authority and Governors for the recruited units (not in Empires; want some buffer in your coeffers since you will be going in the large income deficits for the war time), Military building XP bonuses and spreading generals in 1 peasant units to provide further regional bonuses (not in Empires; not so much against AI cuz you will barely have any losses with a proper stack composition),
Trade system that basically doubles region values to allow you to maintain even larger forces. Diplomatic channels to exchange statuses, units and resources. Kind of alternative often forgotten source of income. Also a source of decent provincial units often optimized for specific terrains that you cant get otherwise without owning those territories. And having them can often mean going over Demesne limit after temporarily holding occupied territories. That beyond obvious listed maluses increase the chance of generals betraying you and governors rebelling.
On the ground Pillaging regions besides obvious economical benefits helps to reduce their Diplomacy value as well, makes religious and ethnical conversions lightning speed - another aspect of long term planning. Retaliation action helps to rack up the War Score and weaken their potential accumulating forces that try to reinforce key points - you have to think in advance where you can hold and retaliate from while restructuring your main stack to meet the new challenges.
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There are more long term angles related to the other systems and exploits, but I promised to keep it short and to the point. I covered all major thesises for the long term planning of the war since the game flow dictated by your forces conquests. If you dont play ambitiously and dont think about win conditions: gaining Legacy through the wars with Legacy leaders while maintaining top tier Legacy passive generation, then you not truly playing the game long term, therefore such weird notions find a way to sneak in your thought processes.