That idea sounds cool! May I suggest you make two forochel factions? They will share some generic troops and characters, having some differences.
Generic Foroches units:- tracker with pack of wolves
- Warriors with throwing spears
- Warriors with stone axes and leather shields (metal breaks easily in very cold climates and are a hell to wear)
- Hunters: able to set traps, like Isengard mines but invisible, able to hurt only units (not walls).
EvilSnow Trolls
Good- Archers in sleds with husky dogs?
Characters- Warchief (Generic)
- Shaman (summon White bear mandatory!) (Good)
- Ice Witch (Evil)
Also, you may wish to reduce movement of all enemy factions when fighting some deep-snow covered scenaries (excepting Elves and maybe some Goblins units).
Hope it helps!