After some play-testing, I settled on some new modifications. There does not appear to be a way to edit old posts on this forum, so I'll just have to repost the details instead of editing my intial reply:
My stat increases will include the basic +10% attk +10% def all COs have. I prefer to see them represented in totals rather than separated. So for example Will has a +20% Offense. But because the generic +10%/+10% bonus all COs have its actually +30% Offense +10% Defense, which is how I intend to list them.
Changes:

Zone: 2
Boost: Ground Direct
Offense: +35%
Defense: +10%

Boost: Ground Direct
Offence: +60%
+2 Move
Will is pretty solid. Only added a tiny attack boost to his basic Zone bonus and a much bigger one to his Power. Now his CO Power is actually worth sacrificing his enlarged zone to use.

:
Zone: 3
Boost: All
Offense: +10%
Defense: +35%
Other: +1 Movement
Defense sucks in this game. I experimentally had infantry attack another infantry to test it. While standing in Woods the defending infantry took 6 damage every time with a 30% terrain defense bonus. Adding Brenner's default 30% bonus on top of the terrain bonus caused them to take 5 damage. So doubling their defense from +30% to +60% made a difference of 1 whole damage... That sucks. When I added modded Brenner's aura of +40% the infantry in the woods took 3 damage instead. That felt better.
His new +1 movement to all units in his zone can do some incredible things. It also fits with his story of defending and rescuing. Always on the move. I dropped his modded defense bonus by 5% to compensate for his new movement bonus.

Boost: All
Offense: +10%
Defense: +60%
Other: Heal all units by 3HP.
Even MOAR defense. Its not like it makes getting your CO power easier the way attack does so I wasn't ashamed to give him this kind of global benefit when he uses his power.

Zone: 1
Boost: Ground
Offence: +30%
Defence: +30%

Boost: Ground
Offence: +40%
Defence: +40%
Movement: +1
Range: +1
Vison: +2
A small boost to her Power of an extra 10% attack and defense over her norm. Helps compensate for small zone and situational vision buff.

Zone: 2
Offense: +20%
Defence: +20%

Boost: All
Offense: +30%
Defence: +20%
+2 Movement
+2 Range
Left the Zone as is. Added only 10% extra attack beyond what her power normally does.

Zone: 2
Boost: Air Units
Offense: +50%
Defense: +30%

Boost: Air Units
Offense: +70%
Defense: +50%
Movement: +4
For something as squishy and easily countered as air, this boost felt better. The +4 movement is remarkably gratifying and the increase in size of her CO zone was much needed. Also bias lol.

Zone: 2
Boost: Indirect & Navy
Offense: +20%
Defense: +10%
Other: +1 range

Boost: Indirect
Offense: +35%
Defense: +10%
Range: +2
Indirect units already have considerable firepower. Gage doesn't need more damage, he needs range! Now any indirect unit within his zone gets +1 range. Quite powerful, but it differentiates him as a CO. The original version of Gage felt bland on the battle field. I am considering reducing the zone attack bonus to 10%.

Zone: 5
Boost: All
Offense: +20%
Defense: +20%
Heal All units by 1
The 1 healing added makes him feel more unique despite lacking a CO power. Also represents that his troops are grizzled veterans and fully dedicated to their honorable leader. As such they are harder to kill. The 20/20 big zone is cool but it wasn't much different from Isabella until I added the minor healing.

Zone: 2
Boost: Air
Offense: +30%
Defense: +50%
Other: +1 Movement

Boost: Air
Offense: +30%
Defense: +300%
I made him the opposite of Tasha, more centered on defense and movement (because he's a f**king egocentric coward). I can't decided whether to increase his Zone size by 1 or give him +1 movement. For now I have it at +1 movement.

Zone: 3
Boost: Navy and Copters
Offense: +30%
Defense: +50%

Boost: Navy and Copters
Offense: +50%
Defense: +70%
Other: Refill all units fuel, ammo and materials
Lets face it. Navy units just aren't everyone's favorite and the default Greyfield sucked. Badly. This version gives you a big enough boost to incentivize the use of his sea units and copters. Remember, navy units are the most expensive in the game, which is the reason I made his bonuses so big. Story-wise this represents his amassed wealth. He can afford the best armor and payloads for his navy units to crush the hated Lazurians across the sea.

Zone: 3
Boost: All
Offense: +20%
Defense: +10%
Other: All units don't get affected by the weather

Other: Changes the weather for 3 rounds.
Offense: +20%
Defense: +20%
+1 Move
Added 10% more attack power to her zone so she can get her power a little more rapidly. The CO Power bump in power only lasts 1 day out of the 3 days of weather. 1 Extra move and extra 10% attk + defense didn't seem like too much to ask. It makes sense that her troops would be emboldened by their invulnerability to weather effects while the enemy is scattered, slowed, and blinded.

Zone: 0
Boost: All
Offense: +60%
Defense: +60%

Boost: All
Offence: +60%
Defence: +60%
Other: Bomb does damage in a certain area, AI controlled, favors total HP damage. deals 8HP to units
Left as is

Zone: 3
Boost: All
Offense: +50%
Defense: +50%
Other: Heals units by 3HP
Tiny nerf. Still potent as hell. Mostly left him that way because he's a reward unlockable who is supposed to kick butt. And since nobody plays online anymore, I fight against him when I want more challenge.