I am guessing, and hoping, Ashlands will have the fire giant Surtr as the boss, i'd love to fight a huge behemoth with large AoE sword attacks. with meteorites crashing down, and the ground rupturing in volcanic blast when he strikes the Earth with his weapon. Then again i haven't a clue where the final boss would be, i'm imagining the Ashlands and Deep North to be equals in terms of progression so perhaps the Ocean would host the final boss, a poetic end as you've been from the beginning sailing through the very waters that would hold the end of your journey, that or the yggdrasil. Then again i haven't explored that area yet so i'm not sure.

I was thinking the Giant Wolf from the mythology would be the boss for the deep north, as the moutains house fenrings, and a cult of wolf-people, so it could be foreshadowing that in fact it will be Fenrir that you'll fight in the deep north.

And why i'm guessing Surtr will be the boss of ashlands is from the surtling enemies, there's a general trend in Valheim that has enemies/creatures in the biome that are basically the lesser version of the boss. Deer into Eikythr, Greydwards into Elder, Blobs into Bonemass, Drakes into Moder, Yagluth is a bit of an outlier, but he's the undead ruler of the fulings(?). Seekers into the Queen (though she resembles a hydralisk for me more than anything, wonder if they got inspiration from the Zerg for Mistlands}. That's what i generally think anyways. Though the Ashlands is being redesigned a little so who knows what it'll be.