I think the worlds in this game would be BEAST, so I thought up some world designs and names.
Jumbled Jungle
The world we first saw. I don't really have any ideas since we've seen what it will be like, but the boss would probably be some type of inorganic monster, since there's a temple.
Chilly Chasm
The ice world we've seen in the artwork. There will probably be a whole bunch of slides, cliffs, and ice, and perhaps it snows constantly? :O You might need to help someone from freezing, and they'll give you a pagie, or get someone's items from one point to another. The boss would be a yeti?
Undersea City
Instead of your usual water level, I want a world where it's a city underwater, with glass domes surrounding the city. It would be like those places in large aquariums, where you can walk down a hall of glass and seeing animals swim around. That would be sick to explore, especiall if you go into outskirts of the city to venture off. Also, I would want a squid boss, not a fish one.
Cursed Keep
Your horror world! I suspect ghosts and paranormal activity here. Maybe the pagies make the world expands downwards, discovering the secret behind the ghosts. MAYBE a ghost boss?
Taxing Toybox
Okay, I would LOVE for this world to be in the game, because it could be made up of building blocks, and since you expand the worlds, it would be such a great idea to make it as if it were Lego blocks! A giant toy soldier that commands the toys into seneseless war with eachother could make an interesting boss.
Stellar Sanctuary
The sky world. Imagine a beautiful view of the night sky and exploring and expanding this giant world with an observatory that unlocked different rooms for pagies. I don't have an idea for the boss at all, except perhaps a giant bird?
What'd ya think?