Ashlands is upcoming, then after that it's Deep North.

Their original plans featured an ocean expansion but that may come after the Deep North. Oceans might be a big project but it's not really in the chronological progression scheme as far as I can tell.

My theory is since the Ocean is omnipresent at any point in the game there will be different levels of reward available to you there. Just like how fighting sea serpents on a raft or karve is a bad idea, but you can later take one down with a proper bow or longboat.
Meanwhile as soon as you have a pickaxe you can gather barnacles from Leviathans.

I think it will be different activities that benefit you at various stages, so it will likely be the last thing on the list of "biomes"