This topic is about several things at the same time so I will cover them in order. These topics range from esthetic to game technicalities.
I noticed that some of the Biomes and environments are very distinct and alien to each other. However I would like to see more exotic plants. I have the feeling that the trees are not quite alien looking, more like a recolor of trees with different kinds of timber and we should see some different shaped leaf blocks, such as palm trees and cactuses.
I would also think it would be interesting to have more elements such as sinking sands and natural blocks with properties such as slowing or flowing properties.
One important topic is the Biodiversity for each one the planets, I would really find interesting to make some species exclusive to some planets, since there is no logical reason of why one species would thrive in one planet from another. Maybe some of the creatures could be captured and trained or traded, instead of only being for raw material farming purposes. Maybe some kind of saddles could be added to some creatures to be able to train them as mounts? I would find a shame if they to restrict them to animal stock.
A note that also would be important to add but that might be in development still is the physics in the world. I think water physics are still on a early stage but I would think it would be very interesting to let water flow, have waterfalls and water physics. Also add some item to be able to collect water and transport it from one place to the other. This would apply to lava as well which should have an item as well to be able to be transported and replaced as a block.
These are my suggestions, hope to hear comments on it and corrections (maybe some of these things are already in-game). I would also like to hear some feedback from people on the forum about these topics.
Have a Nice Day!