Ys/WMG

Feena and Reah stole the black pearl
Little details scattered around the Ys-verse would seem to indicate that Feena and Reah, far from being wise goddesses, were a pair of rebellious teenagers. Mask of the Sun tells us that the black pearl originally came from Celceta, and the Tabulas from Ark of Napishtim hints that the black pearl is a special Unobtainium called Emelas. They also tell us that Humans + Emelas = Big Trouble, and unfailingly results in uncontrollable monsters. Feena and Reah would either have to be evil or dumb to give this power to a bunch of humans.

So here's the theory: Feena and Reah were teenagers by Precursors standards living in Celceta, and believing that humans should be cooperated with rather than eaten, they stole a big chunk of black emelas and fled to create a Utopia where they could live together in peace with humans. It didn't end well.

Lilia is also a descendant of one of the six priests
When you get Lilia's medicine, her mother gives you the return spell, the magic of Priest Hadal. This apparently had originally belonged to Lilia's late father. Why would a man in Rance Village have Hadal's spell instead of Hadal's heir in Lava Village? Because he was also a member of the Hadal family, from a different branch. This would also explain why Lilia was drawn to the core of Ys along with the Goddesses and the six confirmed heirs. The reason there weren't even more people traveling there is because Dulles was making a deliberate effort to purge the bloodlines of the priests with his sacrifices, and he succeeded in killing most of them.

Kishgal is Tyalmath
In Ys Origin, all of the bosses from Ys I and II are accounted for, except for two: Tyalmath, the boss of Noltia and Druegar, the first boss of Solomon Shrine in Ys II. However, Kishgal--one of the Darklings that you fight on Eunica and 's paths, likes to shoot out waves of ice spikes much like Tyalmath. Seeing as, it seems likely that the demonic influence finally got the best of Kishgal.

Galbalan was created by the Eldeen
One of the bosses in Ark of Napishtim (The second dragon) is referred to as the original Galbalan. This is the name of the Big Bad in Oath in Felghana (Though they don't look anything alike), whose few spoken lines make references to the Eldeen. Theory: The Galbalans were created by the Eldeen, possibly as guard creatures. The one that attacked Felghana was one that escaped and went rogue after the Darklings stole the key to the Ark, causing the end of the Eldeen civilization.