For a video game I will make in the future, I am for now trying to write out the entire story for it. However, it ended up being an extremely long fantasy epic. For all the events, story arcs, and character development, it spans across an extremely long time length. For more details:
Sidequest storylines don't count, as this is for the main story itself
The story here is for one game itself. The reason it's not split up is because, if it becomes a franchise, each game would be like how The Elder Scrolls or Assassin's Creed are, each game taking place in a different period of time
Throughout the story, the protagonists have different allies and relationships, resulting in many different characters throughout
The story does not take place in one nation or kingdom. In fact, it is in about 3-4 continents
What are some tips for handling a story/narrative that is extremely long, keeping the character arcs and development not getting old?