So you've finished all the biomes and bosses currently available in Valheim - what now? Well, you could wait for the next official update from Iron Gate: but the next biome is quite a few updates away, so you may be waiting a while to explore the Mistlands in all their glory.
Alternatively, you can just install a bunch of mods. Here's a video of me exploring a rejuvenated Ashlands, and running away performing a tactical retreat when I realised that the enemies are actually quite difficult. Thank goodness I had a cheat sword handy.
Over on Nexus Mods, it seems several modders have taken it upon themselves to populate the game's currently-unused biomes. Areas like the Ashlands and Deep North are currently explorable in-game, but there's little for intrepid Vikings to find there (as these biomes are still in development - Valheim is still an early access game, after all). To solve this problem, modders have introduced new enemies and environments to keep you on your toes.