Once your character reaches max level (level 60), you can visit an NPC in the Caverns of Time e.g.. Chromie and then she resets you character back to level 1 and teleports you to the starting zone of your race where you can choose a new class to level in if you whish.
Unlike a new player, you would retain your gold, pets & mounts collected. While leveling back up again you can choose another challenge to add, e.g. the first time you leveled you selected Slow & Steady and this time you want to try Way of the Samurai combined with Brewmaster etc. etc.
When starting over again the character could get an automatic +1% Crit and +1% Hit buff as a reward or maybe other stats are increased like strength if you are a warrior etc.
Rewards after completing the challenge and leveling back to 60 might yields things like donation tokens, exclusive transmogs, exclusive mounts and titles
Just an idea/suggestion
