allow the player to recruit by regiment rather than brigade
I am already doing this in a mod. There are a few issues with it. Assume you don't alter anything except the units you can recruit. For this example, we won't count the elite brigades (Excelsior, Mississippi, etc), and we change all the brigades to regiments.
Managing the stack panel can get very cumbersome. I don't know if it will affect AI turn processing. I have a fairly good machine, exceeds the stated requirements for CW2 by quite a bit, and even on a vanilla CW2 setup a large stack will slow down the game.
Under the vanilla setup, a "division" requires 4 CP to be effectively commanded. Since there aren't any (or many) brigade units, every "brigade" has to be formed by creating a "division." Most brigadiers out of the chain of command will be at a disadvantage (only 2 CP for a standard brigadier out of command chain).
My solution is to create 1-3 element units for infantry and 1-2 element units for cavalry. There will be a few 2 and 3 element brigades per state. None of the standard units require over 2 CP. The elite brigades are altered to be almost all 3 element units that only require 2 CP. Their unit definition allows them to add another infantry element in the same way a vanilla CW2 militia unit does, so you can end up with a 4 element unit that only costs 2 CP and only requires line replacements. Artillery are all single element units, although I am knocking around the idea of a couple of artillery brigades.
But on a CW3 setup, they'd need to adjust stack panel management with the potential for many more large, unwieldy stacks. I'm just telling you, Joe User, this; I'm sure the devs already know this kind of thing.