Interesting ideas. Making me sit back here wondering what would be plausible and what would be neat while trying to imagine where the line is drawn from the developer side.
Basically put:
- Alpine, Arctic, Coastal Rainforest, Desert, Jungle, Prairies, Savanah, Swamp, Temperate Forest, Tropical, Tundra (cold), Wetlands
That's a nice number of them right there. I'd love to build in both the tropical and arctic places. We missing anything? With how fruits and veggies should be biome-locked so that it would force people to venture outwards and how it'll encourage people to use trains and such to transport them around. Seeing people become hermits in Minecraft irritated me so much that I want to see how it encourages people in this game. I love the selection suggestions for each biome. Something to make note of now are ghosts along with flying birds and other flying creatures.
I would love it if there were two ghost types to add in, one normal type for warmer places and more spooky types in cooler arctic places. I wouldn't mind if we even had "yuki-onna" (snow lady) added, but that might be too freaky and more for mods. Would have to be quite rare (full moon) type deal if we did add one. A yeti wouldn't be bad though. Not sure about undead pirates....That's close to zombies, and we all voiced out opinions in how we don't want those.....Tropical hostiles should be something along the lines of......Good question.
Ya, I don't know. I"m stumped. I guess I have to wait for the trio devs to tease and taunt while giving out hints and surprises. My only wish is that I can tame either a fox or an arctic fox 
(Companions I want shall be left for modding so I'll now go silent about that.)
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Edit: I guess for both snow & tropical they should add snow and sand crawlers - http://imgur.com/a/veFjC - I was just linked this so that may suit the game more. How to defeat them? Smash them up or dry them out in the sun.