

A rivalry so good, you wish there was more
I think Kai vs. Morro is the most interesting rivalry in the series, even over Morro and Lloyd itself in Possession.
Plus this is the season where Kai finally sets through with what he said about protecting Lloyd in season 1. You'd wish at some point Kai just mentions someone going crazy over the Green ninja is so annoying in itself and everyone blankly staring at him like he wasn't incredibly annoying about it for 2/3 of season 1.
But back to Morro vs Kai. Obviously there's the fact that they're foils and Morro is what Kai would likely become if he never stopped to realize he just wasn't the Green Ninja, if not worse.
The entire season could've driven deeper about how obsession drives you to do things you'd never imagine or even more scenes where Morro is incredibly manipulative.
And knowing Kai has good intuition, but bad at bargaining, Morro could single him out and even manipulate him to sabotage the team. "Want to protect your friend? Sure. Just make sure to stop your friends from following us." Or even Kai could just feel so guilty about losing Lloyd his judgement of wanting to do the right thing could make him do the opposite entirely. Hell if you wanted, he could still feel some guilt about Zane.
This could've pushed Kai to his limits and instead of his old obsession with wanting to be the green ninja, he could become so obsessed with protecting Lloyd, that he'd actually accepts Morro's conditions. Like could you imagine if to some degree Cole becoming a ghost was Kai's fault or Kai "missed" his aeroblade and made one of the ninja unable to reach the moving ship piece during the chase down the mountain? Better yet if he made himself a ghost on accident to fully cement the foil between Kai and Morro, but showing the difference is Kai became a ghost through sheer selflessness and even if he did sabotage the team, he would never put them in that level of danger. In contrast to Morro actively putting himself and even Wu in danger before becoming a ghost.
This also gives Kai a call back about his dislike of water earlier this season and now he has all the reason more to be afraid of it. This is also a good callback about Kai also becoming a green ninja(ghostly hugh) like a pattern of when he wore the suit during his secret training.
the rest of the team recognizes he's just reverting back to his old self trying to ride solo forgetting Lloyd has other protectors and friends too and what Kai has done up to this point has been incredibly reckless.
Plus there's just the fact that Kai doing what he does could be what sets Morro on his own internal struggle to understand why someone would go out of their way to sacrifice everything to save their friend, leading to his own eventual descent into betraying the primordial and still being dragged and giving his speech about how you can only save those who want to be saved, it could even have the same power if he said it to Kai, but Wu is good too.
Idk feel like that has incredible au potential and probably just way too dark for what Ninjago would later become, but the first 5 seasons kinda had great horror elements. I think season 5 going all in would be interesting.
I think there was just more to dig at with Kai and Morro that shows 2 sides of the same coin, but Kai having to understand he isn't the main character of every scenario and knowing when to stop. This could also mirror Zane's sacrifice, but presented as far less noble and almost gross in a way since Zane did his sacrifice out of love, while Kai has always struggled with main character syndrome.
Damn, that's a lot to read. Sorry about that.