Random Leveling Dungeon Queue.
I love this in FFXIV because it keeps ALL the dungeons alive and relevant and helps new players find groups faster for their dungeons. All they would have to do to incentivize it but not make it feel mandatory is offer some Valor or something as the reward for doing a few of them per week.
Have it work like time walking, and sync you down to the level and gear for the dungeon or like Normal mode sync where people's healing damage etc is reflective for the level.
I just would love to do some of the old dungeons again outside of Timewalking and I enjoy helping new players out but the only way I can help them in dungeons is to go in on my max lvl toons and do it for them. Its not helping them learn.