Each PC will only have one parent in common. So player 1 is the child of the Serpent King and the Human Queen. But player 2 will share either one of the parents in common, but have another different one: if human then a monster/animal/mythic creature; if monster, then a human parent.
The non-human half will give the player a non-numerical ability. Then they set off on a hexcrawl across Ynn in search of the Serpent King and the reason for Ynn's malfunction. Try to attempt to use all the monsters from the JFMM ("jay-fam"?) and give it a very fairy tale feel of the Bros. Grimm variety.
Maybe some of the antagonists should be basilisks or behirs who were lizards grown monstrous from wearing the Serpent King's shed skin like a false crown.
Very relevant to this effort: https://blog.thesconesalone.com/2019/03/the-gardens-of-ynn-referee-control.html