Here's my slowly-updating mod, started a long time ago. The original idea was to make a mod based on Kalevala, the Finnish epic. It's since changed into a mod loosely based on Finnish mythology in general.
There are many omissions. It uses Bogarus flag, the nation has no description etc etc. There are some minor changes in the 0.87 version, but the only important ones are that Sampo is now properly unique, and that old saves are now incompatible because I increased the unit numbers to avoid conflicts with other mods (Conceptual Balance 1.3).
Magical access: good Fire, Nature; mediocre Death, Water, Earth (summons); good heroes.
The best mages are young, recruitable anywhere and vulnerable to magic
Troops: Stealthy hiisi (=vaetti) and human infantries and archers, stealthy moose cavalry, fast vanir-like cavalry with magical attack.
Priests: weak, stealthy, all Nature mages can spend gems to cast Fanaticism (Spellsong of the High Heart).
Here's the thread where I originally posted my ideas, if you want to see how the mod developed. It's not that interesting, IMO.