I thought having fluid rock is the most unique and interesting alternative to the other 3 unique placement mechanics. There is loam which creates curvy earth walls, a wall kit for perfect flat granite ceilings/walls and the Basement.ocd object which has no material at all but is a strong building which takes more damage than 2 firestones. A different version of any of these ways to build a wall could make one of these 3 items useless. E.g. if a rock wall kit is cheap, maybe nobody will use granite/metal wall kits or the other way around.

@All posts: I see that a fluid rock item could have not enough use cases. One of the few "better use fluid material" cases that come to my mind would be if a lake freezes and you throw some concrete on it, then you have a super flat, safe bridge on the water (rare case). Or if you want to make it hard for the enemy to dig out a cave to your base but the cave is too small to build a Basement.ocd in it (happens in CoFuT sometimes). In CR, fluid stuff was also a cheap way to fill tiny holes against acid but there are e.g. less holes in OpenClonk settlements because you can build several overlapping Basement.ocd objects.