Mistlands, besides what I have seen in videos, (new mushrroms, enemies, Crossbow, Bone Bow, Great Sword (looks awesome btw), Bear Traps (implying bears maybe) Giant Ticks, Hare, Flying bug and the new Marble build pieces as well as new crafting table) I'm guessing there might be at least another 2 or 3 enemies that haven't been revealed yet, and I'm guessing it might be Elves or something.
There would also probably be either more of the same or different enemies again in the Cave thing they put up with the webs in Mistlands, again though, at a guess.

I've been exploring the Mistlands on one of my mining worlds too, and the weather is the only thing that seems to be adjusted from the old variant that had the giant trees in it - It's misty, and baron.

But, from the teaser shots I seen and the small glimpses I have seen of it, it's starting to look like a bit of a combination of Black Forest, Plains and something completely new all together, so looking forward to seeing that, actually glad they changed the landscape.

Also, wouldn't mind having some of this included in the update when it gets released, if it hasn't been bought up already.

1. Buildable Ivy would be sick.
2. Plant-able Red Mushrooms/Cloudberries (cloudberries only in plains though)
3. more weapons, maybe more Obsidian type things. Obsidian Spear?
4. Build pieces with Ancient Bark please, maybe mossy roofs. I have tons of it.
5. Basket Weave Fences (wood) Stone chest (stonecutter) small storage barrels.
6. Another recipe for Serpent Meat, Maybe combined with Plains tier food.
7. Another recipe for Fish?
8. More furniture obviously, hanging deer & wolf rugs would be a good alternative.
9. More shield paint varieties & patterns.
10. Fenrings shouls drop Fenris Hair, (to make it renewable) the only unique thing they drop is a trophy currently.
11. More enemies in Caves & Crypts maybe add leeches in the water or something.
12. An agressive fish that bites you when you are wading/swimming.
13. Eels. (catchable with fishing rod)
14. More stuff for Haldor to sell.
15. Armour set with Yimir flesh because why not. Berserker Armour? (buffs melee)
16. Add Bears in one or 2 of the biomes.
17. A new Weapon for the new forge? Maybe craftable Drynwyn with Flammetal?
18. Black Metal Great Axe, Black Metal Mace, Chitin tipped arrows? Chitin Spear?
19. Pine cone drop rate increase would be good.
20. Increase Serpent day spawns. (they do spawn, but rare as hen's teeth)
21. Add new points of interest in all biomes?
22. make 1 & 2 star Lox to spawn in the wild.
23. More recipies for the Oven
24. More enemy types in the Meadows.

Off the top of my head for now. (there might be more)