You could fairly easily mix in the OOB with the front mechanics.
1) Three levels of subdivision: theatre, army, corps. Theatres work roughly as they do now, you can put whatever you want in them. They're intended to represent huge fields of operations. Corps are intended to represent a unit tasked with a single specific task: for instance, protect a few provinces of the front, take a specific city, garrison a province, etc. They can only be assigned 4 (or some other arbitrary number) divisions without penalty. Somewhere in between, armies are units tasked with specific strategic objectives. They can contain a maximum of 6 corps (or another number) without penalty; they can also contain individual divisions, counting as half a corps. In total, without penalties, that means they can have a maximum of 24 divisions within maxed out corps, or 12 individual divisions. Armies exist only within theatres; corps can also be part of a theater or an army. Theatres can also lead individual divisions.
2) Three types of leader: field marshals, generals, lt generals. Lt. generals can only be assigned to corps, generals can be assigned to corps or to armies, and field marshals can be assigned to whatever you want. Promoting generals costs political power, but doesn't affect skill. (If you want to make all you leaders field marshals, have fun with it, it's just a waste of resources.)
3) Two types of leader traits: logistic trait = current marshal bonuses, and combat traits = current general bonuses. Only marshals can get the former, while lt. generals and generals get the latter. However, marshals don't lose the traits they already had upon being promoted: they just don't use them when they leading a theatre. Combat bonuses appy to the leaders of armies and corps. Skill/trait bonuses apply fully to units being lead directly, such as individual divisions in armies and theatres. This incentives making a full chain of command, but allows you to also keep a number of divisions at the army/theatre level, for instance to use fewer generals for insignificant tasks such as garrisons, or to act as reserves.
You could also introduce a factor that slows down XP generation for field marshal past a certain number of divisions, so that you're not penalized for making sensible theatres as opposed to stacking all your troops in a huge theatre of doom.
4) No HQs, because they just create micromanagement and don't help. If you want to create a ridiculous army with units all over the world, have at it.
5) Now, the radical part: you can give theatres, armies and corps specific battle plans. Lower level plans can either be standalone plans, or they can be part of higher level plans. The lower the level, the higher the priority. For instance, if you have a corps plan that's part of an army plan that's part of theatre plan, once your corps finishes its corps-level plan, it will start acting as part as of the army plan, and so on. You can assign theatres and armies any number of plans you can endure to micromanage, but corps can only have one -- just as divisions can only have one order.
For instance, say you're Germany in 1939. You want to invade Poland roughly following the historical von Manstein plan. Your theatre plan could be something like "take all of Poland". Let's take 3rd Army, in East Prussia, and give it a plan to move straight South towards Warsaw, as part of the theatre plan. Within that army, you create four 2nd and 3rd Corps, and assigned to the army plan. 4th Corps is next to Danzig: you give it a corps-level plan to take Danzig, and put it under the army-level plan. 1st Corps, on the other hand, is protecting the Eastern part of the frontier. You give it a specific corps-level plan to cross the frontier and move towards Grodno so it starts accumulating planning bonuses, but not assigned to any Army or Theatre plan. You intend to activate this plan later, during the mop up phase.
On day one, you activate the theatre-level plan. This activates all plans which are under this huge "take Poland" plan, including the one you gave to 3rd Army. As such, your units in 2nd & 3rd Corps move towards Warsaw, while 4th Corps takes Danzig. 1st Corps stays put, because its plan was not under any other plan. Once Danzig is taken, 4thCorps goes help 2nd & 3rd around Warsaw. After the order is completed (and, presumably, the Polish army was surrounded around Poznan and annihilated), they all start moving East, taking whatever's left to take. You then activate 1st Corps, which starts moving East as well -- you could also assign it to the theatre plan so it keeps doing stuff after taking Grodno.
You could also make it so parts of a theatre front can be assigned to armies/corps, and army fronts to corps. This way, when your front line is not balanced, instead of the battle plan AI only redeploying units accross the map (i.e. transfering divisions from a corps to another), it could also adjust the frontier between units, i.e. progressively move its troops parallel to the front to fill holes. Heck, you could even put a "balance front" command just
This would require significant amounts of coding, + thinking about ways to make the UI intuitive, but then again the current battle plan system also requires that anyway.
1) Three levels of subdivision: theatre, army, corps. Theatres work roughly as they do now, you can put whatever you want in them. They're intended to represent huge fields of operations. Corps are intended to represent a unit tasked with a single specific task: for instance, protect a few provinces of the front, take a specific city, garrison a province, etc. They can only be assigned 4 (or some other arbitrary number) divisions without penalty. Somewhere in between, armies are units tasked with specific strategic objectives. They can contain a maximum of 6 corps (or another number) without penalty; they can also contain individual divisions, counting as half a corps. In total, without penalties, that means they can have a maximum of 24 divisions within maxed out corps, or 12 individual divisions. Armies exist only within theatres; corps can also be part of a theater or an army. Theatres can also lead individual divisions.
2) Three types of leader: field marshals, generals, lt generals. Lt. generals can only be assigned to corps, generals can be assigned to corps or to armies, and field marshals can be assigned to whatever you want. Promoting generals costs political power, but doesn't affect skill. (If you want to make all you leaders field marshals, have fun with it, it's just a waste of resources.)
3) Two types of leader traits: logistic trait = current marshal bonuses, and combat traits = current general bonuses. Only marshals can get the former, while lt. generals and generals get the latter. However, marshals don't lose the traits they already had upon being promoted: they just don't use them when they leading a theatre. Combat bonuses appy to the leaders of armies and corps. Skill/trait bonuses apply fully to units being lead directly, such as individual divisions in armies and theatres. This incentives making a full chain of command, but allows you to also keep a number of divisions at the army/theatre level, for instance to use fewer generals for insignificant tasks such as garrisons, or to act as reserves.
You could also introduce a factor that slows down XP generation for field marshal past a certain number of divisions, so that you're not penalized for making sensible theatres as opposed to stacking all your troops in a huge theatre of doom.
4) No HQs, because they just create micromanagement and don't help. If you want to create a ridiculous army with units all over the world, have at it.
5) Now, the radical part: you can give theatres, armies and corps specific battle plans. Lower level plans can either be standalone plans, or they can be part of higher level plans. The lower the level, the higher the priority. For instance, if you have a corps plan that's part of an army plan that's part of theatre plan, once your corps finishes its corps-level plan, it will start acting as part as of the army plan, and so on. You can assign theatres and armies any number of plans you can endure to micromanage, but corps can only have one -- just as divisions can only have one order.
For instance, say you're Germany in 1939. You want to invade Poland roughly following the historical von Manstein plan. Your theatre plan could be something like "take all of Poland". Let's take 3rd Army, in East Prussia, and give it a plan to move straight South towards Warsaw, as part of the theatre plan. Within that army, you create four 2nd and 3rd Corps, and assigned to the army plan. 4th Corps is next to Danzig: you give it a corps-level plan to take Danzig, and put it under the army-level plan. 1st Corps, on the other hand, is protecting the Eastern part of the frontier. You give it a specific corps-level plan to cross the frontier and move towards Grodno so it starts accumulating planning bonuses, but not assigned to any Army or Theatre plan. You intend to activate this plan later, during the mop up phase.
On day one, you activate the theatre-level plan. This activates all plans which are under this huge "take Poland" plan, including the one you gave to 3rd Army. As such, your units in 2nd & 3rd Corps move towards Warsaw, while 4th Corps takes Danzig. 1st Corps stays put, because its plan was not under any other plan. Once Danzig is taken, 4thCorps goes help 2nd & 3rd around Warsaw. After the order is completed (and, presumably, the Polish army was surrounded around Poznan and annihilated), they all start moving East, taking whatever's left to take. You then activate 1st Corps, which starts moving East as well -- you could also assign it to the theatre plan so it keeps doing stuff after taking Grodno.
You could also make it so parts of a theatre front can be assigned to armies/corps, and army fronts to corps. This way, when your front line is not balanced, instead of the battle plan AI only redeploying units accross the map (i.e. transfering divisions from a corps to another), it could also adjust the frontier between units, i.e. progressively move its troops parallel to the front to fill holes. Heck, you could even put a "balance front" command just
This would require significant amounts of coding, + thinking about ways to make the UI intuitive, but then again the current battle plan system also requires that anyway.
