A Builders DLC would be phenomenal but just for general game play, internal walls for each Tier. So 4 (sandstone, stone, wood, black ice) walls.
Arena needs sloped pieces. There are some difficult pieces to fit that require using other pieces from the build sets, and adding the addition sloped tunnels to go with the curved ceilings to go over the stairwells, and if you use ramps…
Make the ramps line up properly. They protrude up over the edge instead of fitting like the stairs do, to the adjoining tiles. Perhaps would require 2 types. One with a beveled edge, and the other as it is currently for joining the ramps together.
It would be interesting to be able to color/dye your floors and ceilings.