I think it is a great idea, but dont take it too far, keep it simple and it would be pretty in line with their focus on rewarding battle plans and less micro. Just get rid of (or reduce) the arbitrary bonus % number for battle plans and replace it with one which will depend on corps/army assets, because a good plan will not help you much if you dont have adequate equipment to support it. If you plan ahead your attack your troops will feel the full support, if they act independently they can't rely much on support, same if they get on the defense suddenly. With defense, if you draw a defensive line with "Not one step back!" you get full bonus for frontline troops on defense but not much for any reserves behind current front lines, or you can set it as Defense in depth which would not give the full bonus for the first line of defense but reserves (2nd line of defense) would get similar bonus as the frontline etc. This would also mean that if you are set for defense, your sudden attack would be without suport from these assets, but could be worth it in some situations (because the enemy is set to attack so doesnt have bonuses as well). Add in commander modifiers and it's golden.
This could also lead to feints and mind games, especially in multiplayer and also be more realistic, win-win
As for the support units, they could be recon, artillery, engineers, TD, anti-air etc each giving a distinct bonus for a different area (so not a flat +5% combat strength increase). Different types would have different characteristics, but to keep it simple just stick to 2 types of each (arty/sp arty, engineers/mechanized engineers...) with similar stats but the more expensive ones would reduce the needed time to get the full bonus (and probably also needed to be researched first). So now you can have flexible but expensive army, cheaper but less flexible or a mix of both.
I would see Corps/Army assets giving a combat modifier complementing the 'Planning Bonus' rather that instead of it.
The rest I think is a great idea.
