A long form story can be kept fresh with different approaches among the story arcs. The challenge is to take an interesting angle that feel organic to the story.
Here are some technique examples:
1-Tonal shift: Some story arcs can be darker and others more lighthearted. Think of it like a rollercoaster.
2-Character focus: Some arcs can focus on certain established characters.
3-Changes in the status quo and character dynamics: This can come in many forms: Maybe the Main leads decide to deal with their romantic tension and started dating, or the team leader dies, forcing them to choose another leader.
4-A successor: maybe the protagonist died or retired, then there gotta be one or more characters to shift the focus.
5-Deconstruction: Actions during the seasons have consequences, and some have effect in the long term. Ex: Overusing the forbidden technique may result in permanent injury or death, saving the world once per season means the hero becomes famous, but that also means the villains know better about his abilities and weaknesses.
6-Subversion of the formula: If your story has a common pattern of events, readers will have expectations. Ex: people may expect the MC to fight the main antagonist of an arc, but maybe the antagonist`s powers are perfect to defeat and kill the MC, so another team member may need to fight him.
Of course there may be way more tricks but those are the first ones that came into my mind.......