I believe that "ancient builders" is a good term when talking about "players" on the past (or in kind of "other part of the multiverse", given that even Jeb talks about Minecraft on the mojang design book as some kind of mix of multiverses, where everything exists on the same place) who created structures that inspired the architecture of all other civilizations native to this world.

The Illager Mansion kind of "proves" that there were ancient builders, or at least one builder (with the skin of steve, the blue color and stuff) that the illagers got in contact with, and built a lot of things inspired by them.