This has likely been suggested many times (please link me to such post if so) but alas my forum search skills are terrible (since I still don't know how to advanced search only one specific subforum):
Suggestion: scale quests upward to the lowest level party member
Elaboration: Player A of level 6 wants to play Total Chaos quest for some reason (favor farming, etc) but suffers XP penalties doing so. Player B of level 4 is coming along for the ride since they have played Hard already and unlocked Elite difficulty. Scaling ensures the quest at normal difficulty will be level 4 instead since that is lowest level party member. Then Hard adds 1 level to that and Elite adds another, ending at quest effective level 6. Not sure how this scaling could be implemented, but Reaper difficulty has buffed enemies and debuffed party so something similar might be possible without having to add new enemy types and attacks/abilities.
Benefit: Both players can now earn full XP since they are not overleveled, and even better they could play even lower level quests at Reaper difficulty since they are not overleveled. This could especially benefit level capped players running out of Reaper quests to do on a daily basis. It's free real estate! Scaling the quest to LOWEST level player also encourages party members to not be too far spread out in their levels. This also reduces how much players try solo Reaper1 since lower level quests will now scale up to their level, adding Reaper on top of this for extra challenge, encouraging group play.