The optional quests offered now are not true side quests. They are still part of the main quest line in the same general area / map progression. I suggest implementing true side quests that have their own tangent storyline. These quest should encourage players to explore different areas and tell a better background story to the setting. They should make you hop around different maps, town and even level zones. Major side quests could even stretch over different level requirements, rivaling like a “Mini Main Quest”. Completely optional side dungeons is not out of the question, potentially giving multiple places to grind within the same leveling range.
The benefit of this is giving more opportunity to tell stories and also fill in extra activities during those long grind walls.