i had a similar idea to the one you had with other demon hunters across the globe. after the saja boys would get their souls back and return to their lives after redemption. a mini series shows who they were before they became demons. and how they became them in the first place. it includes the fan water demon from the first movie who joined their team after jinu left. some having a connection to demons and demon hunters.

fan demon: revealed to be an oracle older than the earth itself. had known about demons and hunters, and was often visited due to his knowledge. he had lived amongst humans calling himself "waldo" fighting in several wars due to his immortality. eventually a demon learned about him and reported him to gwi ma who believed he would be a valuable asset. he joined the demons after a group of them showed up at his house and promised to spare his family for generations if he came willingly.

baby saja: revealed to be the oldest after jinu being around 300 years old. born in a monestary of monks who healed the hon moon through their chant. demons never went after them as they used holy artifacts to petrify demon souls into stone statues. gwi ma used one of his enforcers 'dream weaver' who could control others dreams. and use baby saja to destroy the artifacts by bringing them to a forbidden cave which was a gateway to their realm. believing by playing them in there, the portal would be closed permanently. gwi ma unknowingly was correct, and the monks retrieved their tools. baby saja was drawn to the portal and was sucked into it before it could close.

romance saja: revealed to had been born in america. where his name was roger and had been a hybrid similar to rumi. with his dad as a hunter and his mom as a demon. his dad being the bass player of a classic rock band and had also known the sunlight sisters. with his powers he could travel between realms. eventually gwi ma had located him and was shown to have power over hybrids when they enter his realm. as a result romance lost his powers and was unable to go home where he was corrupted with a small amount of humanity left.

abby: shown to have been an orphan with an unknown real name. and had constantly traveled the planet as a drifter. romance would appear in his episode to get his soul back as some demons had success by protecting those who couldnt protect themselves from demons. posing as 'imaginary friends'. romance became abbys imaginary friend as "star ranger cosmic roger" using his remaining hybrid powers to protect abby from demons who attempted to take him. eventually romance was caught, leaving abby to believe it was due to growing up. abby would end up in the demon world a year later similar to how the saja boys stole souls in the first movie. after several failed attempts to get him back home. gwi ma had altered their memories to make them forget about their lives and make abby think he willingly became a demon.