In terms of what I writ here, I more considered them as a plot device, but they also are a hybrid of the big 3 races you make comparisons to.
For the Fay I was thinking of a glass rapid fire support as the core army, that come with high dodge, melee and range base stats.
With the sprites distracting and disrupting, by tying up archers and such by gnawing at them whilst evading harm with their high dodge chance, and/or ability to bypass an engagement hex.
The ents would be less swarm like and of one life unlike the undead, but with swing attacks and extreme durability.
The piper would be more of a target and would be buffed with spells to stun and ensnare enemies, something like;
-captivating tune: halts nearby foes
-sharp note: Hurts surrounding (axe swing)
-ensnaring music: Vines ensnare foe
-rousing melody: Enables ents to act
A unit I didn’t draw was the Siren that would be a beautiful elder that can control minds with song xcom style.
With Burrow-Men, they’re essentially bandits with a stronger core, great range resistance (cba to draw shields), and a bonus for night time combat, high stamina and poor speed + daytime disadvantages.
The sling would have a stun chance, and i’d was thinking they’d need a long-range caster unit to force the enemy towards them, like a little roman catapult to fling loose shingle at you, or a slowly winding ballister (goblin-esk I know lol)
In terms of bringing something simple but unique, I’ve thought it might be cool to have a large enemy that is it’s own army, requiring many battle-brothers to take down as part of the beast faction the werewolves are a part of, and maybe adding a little bit of a monster hunter flavor to your missions, i.e. slay the dragon. I’ll probably post them as a separate suggestion once I’ve farted around enough with my manticore and dragon, but something like this…
And some simple quirks might be…
Manticore: Can sting from 2 tiles with poison dmg, can charge, weak range defence
Dragon(maybe a different name): Can vomit fire onto nearby hexes, can charge/glide
Cockatrice: slow with high armor-dmg, can charge/glide
Wyrm: High ambush chance, hidden on invisible on “campaign map” residing in the marsh, can burrow in bog water
.. on and so fourth idk