Humble Building Bricks
Every module builder will someday have to build a wall in the middle of a room, block a tunnel with a rock pile, make a custom pier, make a frame to add a door to this irregular shape mountainside, invent a special ruin, build fortifications, assemble a credible giant water puddle... And sometimes you need to hide tileset unwanted parts with handy placeables too.
This is the moment you realise you have only one wood log and one post, no planks, no bricks, a few rocks or dirt patches in the standard placeables set. The situation worsens when you discover these are NOT modular, no provisions where taken to stack placeables in the Z vertical dimension and joining placeables moving them in the X, Y axis is poor at best.
What i suggest:
Simple construction placeables parts: Rocks, wood beams, planks, bricks, wall fragments, stone floors, roof tiles, fountain modules, table parts, light effects, column chunks, column tops and bottoms, wheels, modular well/hole kit, etc.
Simple things you really can stack / assemble to build complex objects the size you like, with raccordable seamless textures when needed.