Mistlands are being worked on atm. From the latest dev blog:

"Aside from that, the team is fully focused on creating the Mistlands. New creatures are given life, dungeons are being planned out, and weapons are being crafted. Designing a new biome is a large undertaking, with many pieces that need to slot together before it can become the complete experience it needs to be, and it will require a lot of iterations and testing before it’s done. The biome is quite some time away still, but we are excited for it and we believe the result will be well worth the wait!"

I personally don't expect Mistlands until late summer and the lava biome sometime next year.