Modular vehicle, space ship, robot parts. Walls and corridors are easier to make, so i would rather have those talented make stuff that is hard to create.
There is also one important aspect to this. For eg. that hero pack on marketplace, it has 5 great models but they do not fit single theme/game. So kind of nobody can fully use it.
That would be much better if it was just pirate pack, or “tomb Lara and friends”. So please do not repeat that mistake and make coherent sets that fit same art style.
And if you decide on what kind of set you are making, please in future make expansion sets. It is hard to buy anything in marketplace because it is very visible that different sets are made
by different people. So sadly even if they have same theme its hard to use them in single game, it will look like patchwork.