Okay, I'll also make a suggestion about the interface.
My idea would be that you select land units in a province as usual. With a unit highlighted, click "reorganize" from the command menu at the side of the screen or something.
Now a new screen opens, showing you all the divisions, armies and corps within that province and their respective echelon chains. You're now ready to freely add, subtract, create new or disband higher echelon organizations, mainly by simple clicking or drag-and-drop means.
This way you'll have total freedom to make up an army. For example, an army could consist of 4 independant divisions or 3 corps each with 3 divisions and an independant division, or whatever.
You'll also solve the present limitations when it comes to exchanging sub-units in the present system.
A corps must have a minimum of 2 divisions (or else it's just a division).
To avoid exploits, command is only exercised by the overall leader, although all leaders will gain experience and add traits to the unit(s) under
their command.
Avoid exploit alternative I: corps organizations are automatically dissolved when merged into an army.
Avoid exploit alternative II: corps organizations remain within an army (for decoration only) but are leaderless.
All divisons enter the game unattached and will be assigned a default leader from the leader list (to prevent leaderless divisions). Replace him if you like (recommended, you'll never have to do it again).
Higher echelon organizations will only exist once you've created them (with the exception of those at start of a scenario). There will be no more 23. Infantry Division/I Corps nonsense, duplicated by every new division in the game. You should of course be free to change the default designation of the higher echelon created at will, just like with the divisions.
What leaders are assigned to what will be given with unit information as you hoover over a province, when you click a certain unit or from the leader list (leaders will not simply disappear from this list only because they are given a command).
Finally, it's great to hear we are just about unanimous about the importance of the stay-with-unit feature. Now, we'll just got to make sure they'll hear us.
Bump!