I believe coming up new playstyles to make new commanders unique is as hard as designing their appearance. My suggestion here is mainly intended to share my ideas on new playstyles that can fit the current war system. The unit I mentioned in example is just that, an example, and doesn't need to be taken too seriously.
1. Glass cannon cavalry:
The melee cavalry designed to hit enemy hard and quickly move away instead of staying in the front line.
Example: Hussar
Compared to Magdalene, Hussars have less armor & HP but more attack and 1 more AP, plus they start with a passive ability that allow them to move after every attack. They can easily one-shot lone enemy unit, but make sure they have enough space & AP to move back before they charge into groups of enemy or they'll be destroy.
Upgrade 1: Maneuvering training (Increase 1 AP + enemy can't counter after your attack)
Upgrade 2: Wedge formation (Increase ATK)
Upgrade 3: Heroic Charge (Gains an active ability "Heroic Charge", a strong melee attack that deals 20% extra damage, cost 0 AP and can move after using, but will also increase your damage taken by 50% for the next enemy turn)
2. Tank + long range
A front line unit that can both tank damage and hit enemy long range units hiding behind if needed to.
Example: Legionary
Melee tank with high armor & HP but low damage. After the upgrade they'll gain a long range ability to either soften the upcoming soldiers or hitting those nasty archers/mages hiding behind.
Upgrade 1: Pilum (Gains an active ability "Javelin", a long range attack that cost all the remaining AP)
Upgrade 2: Testudo (Increase DEF)
Upgrade 3: Reserve (Gains an active ability "Reserve Swapping", heals 50% HP, can only be used once per battle)
3. Long range duelist:
A bit like "Tank + long range" but more focused on attack. They're long range units but with some armor that helps them survive enemy archers or can be used as "emergency front line" if enemy breached your tank.
Example: Crossbowman
They wear medium armor that makes them less squishy than most long range units and can deal respectable long range damage. But the overall DPS is a bit less than Ailluin & Jessy in late game.
Upgrade 1: Pavise shield (Increase DEF & M.DEF)
Upgrade 2: Arbalest (Increase ATK)
Upgrade 3 : Fortification (Gains an active ability "Fortification" that works like Worilla's version)
4. Debuffer:
A mage that, instead of dealing damage directly, specialize in decreasing enemy's stat from far behind. Especially useful against high level enemy.
Example: Witch
They have less attack than fully upgraded Bristol, but they'll gain the ability to "curse" the enemy to make them squishier or prevent them from hurting your ally.
Upgrade 1: Stamina Exhaustion (Gains an active ability "Curse attack" that -20% enemy ATK, cost 1 AP and has 4 tiles range, each enemy unit can only be applied once)
Upgrade 2: Armor Corrosion (Gains an active ability "Curse defense" that -20% enemy DEF & M.DEF, cost 1 AP and has 4 tiles range, each enemy unit can only be applied once)
Upgrade 3: Potent curse (Increase the effect of every curses)
5. Late game powerhouse:
At first they're extremely weak and upgrading them cost more resources than usual. Once they're fully upgraded, however, they'll become absolute monster.
Example: Musketman (I'll say it again, this is just a example. You can always replaced it with other units if you have other plans for guns.)
They only have light armor and their basic attack is short range bayonet that deals low damage. They start with active ability "Fire!", long rang attack that deals 2x ATK damage but cost 2 AP and can't move afterward. Upgrading them cost 2X more resources but once you do, they'll become your best DPS units, even better than Ailluin & Jessy!
Upgrade 1: Shoot-and-Scoot (Allow them to move after using "Fire!" ability)
Upgrade 2: Refined powder ("Fire!" now deals 3X ATK damage)
Upgrade 3: Speed loader ("Fire!" now cost only 1 AP and can be used twice per turn)
I hope my ideas gives you some inspirations. :)