I need to speculate!
1) Terraria team is working on Terraria while other team is working on this new game coming out.
2) We were shown three pictures. A desert with a floating Island, a jungle with a sword on the ground and trees/crystal under a sun.
3) The other studio either drew the pictures we see right now or Terraria team created those pictures not only for us but to inspire the team.
4) In the teaser it showed both logos going purple/being changed... Evolving? Being infected? They look like gain veins and odd fleshy looking growth... It was triggered by the flash of light.
5) We can a giant "planet"/forcefield in horizon in the desert picture. It might mean we can go there or it might be just background.... If its a force-field it might mean a safe zone like a city. More NPC? That floating abandoned looking house structure is either a ruin of that or a recently destroyed house from a war?
6) The jungle has water in it and mist. We can only see one sun... Plenty of plant life.
7) the other desert looking area has crystals, a few barren looking trees and some shrubs. We can see a light source but it might not be the sun. It could be underground and be like some area from journey to the center of the Earth. BTW, I'm surprised no element of that has been added to Terraria.
8) crystal desert place has water/liquid. Chemical looking gases are rising up from it. It might be hot there...
9) This game might not allow us to freely change the environment. It looks like it will be more 3d than Terraria.
10) I actually doubt this is the sort of art style will be used in the game. Its really just to give us a vague idea of what it will look like.
I'm going to speculate late more after more people comment or I become more inspired.