How to keep people doing lower level dungeons and questing? Classic +

One of the biggest issues with classic is eventually when new players come to join the server there is not alot of people leveling with them.

Now what could be implemented that made it worth leveling another character or even if they added something that reset your character to level 1 and allowed you to do all the quests again? Like a buff or the ability to transmog or a mount? There's got to be something.