Disclaimer - This is just for fun, should not be mandatory and you can disable it at any time, it's for people who want the questing experience to be dangerous and immersive.
The basic idea (open to discussion) is as a lvl1 character you can enable hard mode leveling which means no heirlooms, maybe no enchants, obviously no exp flask, you can only do a dungeon if you manually go there yourself, the experience you gain from monsters and quests is reduced and you take more damage (not from players)
There is 3 difficulty modes:
Medium - exp reduced 30% / you take 10% more damage
Hard - exp reduced 50% / you take 25% more damage
Insane - exp reduced 75% / you take 100% more damage / you have to do 90% of all quests in every area
If you manage to get to max level with these penalties in place you will be rewarded with an achievement/feat of strength and a title for insane, a starter set of gear, a heirloom for a gear slot like neck/wrist/feet etc
It's something I've been thinking about after leveling a few characters, I think one of the things veteran players miss a lot is the sense of progression and danger while leveling.
Before cataclysm and maybe wrath, I enjoyed leisurely leveling up a character, reading the quests/dialogue, being immersed.
And you all know what it's like now, slap on a few heirlooms, dungeon spam and do like 5 quests in an area before they turn grey, it's fun (sometimes) but not immersive.
This is just a very basic idea I have thought of, numbers are not set in stone and would be up to blizzard to test and iterate.