I'm sure the suggestion of a Skolver/Vog boss has already been mentioned many times, but I thought I'd just throw this out there anyway.

There can be two distinct bosses, a Skolver boss and a Vog boss, and therefore two distinct paths which you could take (freeze path for Skolver, fire path for Vog), each with its unique loots and monsters.

Now, these two deities or demi-gods are nemeses of each other, so they will help you clear obstacles on your way and fight whichever boss you decided to go up against (e.g. Vog spirits can help you melt icy things).

For an epic boss fight, maybe the two deities will go up against each other and you the knights will assist in whatever way you can. Maybe there can be minions of each swarming each other, and you have to ensure that your side is winning while the two big daddies are throwing fireballs and icicles at each other and duking it out. Perhaps, only the spirits can harm the Skolver/Vog boss for the most part, and you need them to do enough damage to put the boss into a weakened state (hence why you should destroy the enemy minions) so you can throw in some blows yourself. On the other hand, if you let the hoards overcome your side, your ally will suffer damage, and if your ally dies, then it is a hopeless battle. Maybe this can be the first use of boss health bars!

I hope this idea entertains some! If not, then I'm sorry I bored you :p

Happy gaming!