Many MMOS have a either
1. A hybrid system where new players start at level 1 with no gear and veteran players start at their previous level. The game would detect their level and only allow them to play at the appropriate level until they've completed the mission, thus unlocking difficulty tiers.
2. A booster for new players to grant them min level requirement and one set of gear to play the content at veteran levels. TTK offered this option via Spark of Light which boosted players to level 25 and fully leveled one subclass.
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The more I think about it, the more that the option to allow players to choose to fresh start or not becomes the route that seems obvious. I nearly started a fresh character in TTK. In a new game I would be even more tempted to experience the new player path at least once.