Suggestion : Restoring Umber and Capitar
As Umber and Capitar are cut from main factions, it might be very fun to add back.
Some ideas I can think of:
1) Capitar
+ special improvement for double gildars generation
+ special unit : mercenary (cheap, soldier but zero in production time, just need to be paid)
+special unit : Night Assassin (fast, deadly but expensive and will serve temporarily for x number of seasons / turn only )
+special hero skill : Bribe (can escape from battle (without scars) or convert lower level human enemies)
2) Umber
+ special improvement: battle arena (scarify 1 hero to level up another hero - the winning hero level addup directly depend on the losing hero level )
+ special unit: Wolfman, a kind of werewolf / beast man, that is strong and can regenerate life every turn in tactical battle but weak to holy magic
+ special hero skill : survival (ability to remove 1 lost-battle injury / defect every level up) - FE Heroes have too many scar nowadays
hope this can contribute some of the points to make our long lost friends in WOM unique and come back.