I never engaged with the main quest and still was able to sink countless hours into this. The Shivering Isles DLC also boosted the play time for me with its eclectic cast, environment and enemies. More depth than Skyrim, but this was the beginning of the downfall of the Elder Scrolls with more streamlined RPG mechanics and progressively less character-driven story. In saying that, the Dark Brotherhood questline is still a huge highlight.