I personally hope that they will add enough building pieces to more then double the current building pieces available.
Some of the things that I would love to see:
Crafting Stations:One thing that would be nice to see is that as you add the upgrades around the different crafting stations that can be upgraded the crafting station itself would have an appearance change for each level.
In Hearth and Home we gained the Iron Cooking Station but we were left with the regular cauldron. An Iron cauldron that could be used for new cooking recipes and was larger in size would be nice.
My final idea for crafting stations currently is to remove all potion making from the cauldron / fermenter and add an alchemy station for making potions. I would also change the meads from the base for potions to buff items like food currently does.
Building Pieces: 1m, 2m and 4m poles and beams available for all materials including regular wood, fine wood, core wood, dark wood and wood iron, as well as adding the 26 and 45 degree angled and cross pieces for all the materials. In addition as
MythicBuilds had said adding the Ancient Tree building pieces would be amazing.
A massive, massive improvement that I think they could make would be to add all of the wall sizes including the angled walls and a roof style for all the materials.
Placeable wood and stone statues.
An in between of the regular bed and the dragon bed.
New fences including a stone one.
Any color available in a torch would be nice to have as a sconce or brazier.
This next one has to be one of my largest wants of all. I really hope that they add armor and weapon display stands. These alone could take builds to the next level.
Last but not least for building materials, it can never hurt to have more tables, chairs, thrones and banners as well as other available decorations.
As far as new lore, mobs, armor, weapons, boss's, mini boss's, plants and general environment goes I'll be the first to admit I'm excited for more of it but could not in the foggiest give any ideas. However I really do like all of the ideas and predictions
MythicBuilds had mentioned in his original post.