Travellers, Other Side, and Arcadius, Sirens storylines could’ve been connected
A theory I posted a while ago on reddit and want to share here too.
While we can say what we want about S8, I wanted to share my thoughts and discuss them with you guys. It's been a while since I (re)watched anything past S4, so bear with me.
So Hell is Cade's dimension created by his severe emotional trauma in what is coined 'Hell.' It's been in existence longer than the Other Side. So before that, is it correct to assume that everyone who didn't have Peace went to Cade's dimension. And before him, it was, only one finding Peace? Or maybe something else next to it? (Lazy ass writing to introduce Hell and a equally badly written villain and even weaker motive)
When Qetsiyah created the Other Side, most Supernaturals went here after they died. I say most because Jenna didn't end up there after her death. She went to what is a version of 'Heaven' since she found Peace. Also, Lexi found Peace after the Other Side crumbled. So it seems that by default, no Supernaturals after its creation went to Hell.
Cade would've noticed something was off. He was collecting souls and could freely enter Limbo, and his Sirens were servants in the living world to punish or kill any doomed souls beforehand. Could it be that when his Sirens tried to kill Supernaturals for Cade 2000 years later, he realized another dimension like his or a force (the Anchor Amara) interfered with his need to collect the bad souls and damn them for an eternity? It wouldn't be far-fetched, I think. Cade wouldn't be able to do anything about it since it is another plane he couldn't reach or destroy to claim all those Supernatural creatures. Not until the Other Side began to crumble was he able to do what he did 2000 years before all souls (i’m ignoring him wanting Katherine that badly to hell instead of letting her pass to the Other Side… it was fanservice to me and gave the writers and opening to bring her back).
Putting aside the inconsistencies in writing and if you liked or disliked the later seasons, Cade could have been built up through the Travelers (the most boring storyline ever) and tied to a common goal: destroying the Other Side. It would've made Cade scarier and built him up some more instead of what we've gotten when he died, and it being revelead that Katherine was the one in charge and pulling the strings.
Cade feels he is owed all these bad souls in the Other Side, and the Travelers want to break their curse and destroy all the Magic/Witches except their bloodline. Cade could've, at one point in time, before the Bennett coven sealed Sybil, ordered her and her sister to see what was up the first moment he couldn't take in any Supernatural souls anymore. They find out a Witch (Qetsiyah) created the Other Side and even come across the existence of the force behind it (Amara being the anchor). It gets an adequately fleshed-out backstory about the Psychics, Sirens, heck, even the tiresome Travelers.... most of Cade's negotiations would be done through Sybil and Selene. This would be a powerful alliance and the balance restored for both these parties that had to deal with the consequences.
This is just my rant/take on how these three (bad) storylines could be done better and have some proper buildup around the characters (I hated it so much after Markos and the Travelers were introduced).
Sidenote: it would've been so cool to see what happened to all creatures when they died before Cade came into existence. We know about the concept of Peace when one dies. I love the mystery surrounding it, and they've briefly shown what it looks like in the Originals a few times in TVD when Elena, Damon, and Stefan died at the end of the season. It's their version of a paradise/Peace. Perhaps a similar concept of not finding Peace could've been mentioned or shown. Would that be Limbo or something like some forever space of nothingness? They did say the Other Side was a concept, but it was only for Supernaturals, not humans. Unfortunately, we haven't gotten any lore that would explain this concept. Maybe it is mentioned in Legacies, but I haven't watched the show, and I won't.