Could also be an additional 'specialization' for each faction, maybe 3 subfactions to each faction.
Orcs: Trolls, Demons, War Machine (Like a moderately fast steampunk battering ram that can fire mortar shells at archery range.)
Elves: Grypphins/Hawks, Beasts, Satyrs
Humans: Gnomes, Man-A-Pult (Launches troops into the air over the enemy castle, taking half health but landing behind enemy lines to do damage. Very effective in numbers.) and Ninjas.
Each can be chosen at some point during the questline.