I recently realized something about the Oblivia Region. Many Pokemon native to Sinnoh live there, while there is an abundance of Johtoian mythology. It got me thinking as I realized the connection is similar to the one shared with the Sinjoh Ruins. Here's my theory:
Oblivia was once one large island off the west coast of Sinnoh. Johtoian immigrants began traveling to this island, as a gateway to Sinnoh (which at the time, Sinnoh was (and still is) considered the Holy Land) some immigrants settled on this island, creating the ancient Oblivian culture, and the events of Oblivian history (the hero that bonded with the legendary beasts, ect...) During this time period, tectonic activity around Oblivia made the islands separate, destroying Oblivian monuments, one of the most important being the Rainbow Dais. To mourn the loss of their civilization, they built the Sinjoh Ruins. They might have also built the ruins to show their alliance between Sinnoh and Johto, Arceus and the Unown.
What do you think about this theory? Do you have anything to add or disprove? If so, don't be afraid to comment!