As far as I know, every default Reign of Giants map has the following:
- At least one instance of every biome
- At least one Pig King.
- At least one (usually 2-3) Totally Normal Trees
- Maxwell's Door, and the five "Things" used to activate it.
What other special game elements are spawned naturally in every map?
By "special", I am ignoring things like type of tree, monsters, plants, boons, etc. Including all of those would make this question far too open.
I am thinking of set pieces or unique items -- things like: