What if level has less to do with achievment and was more about build? IE, you can play to cap in a couple of days easily, but that doesnt mean you're the most powerful being in the world. Really, the game isnt designed to make leveling a big part of it, and even under-level players can take on challenging content with the right skill.
There are two competing schools of thought to consider. The first is the "play to cap and park" system, and the second is "reincarnation". The latter turns the entire game into the end-game, making more content relevant; while the former requires an exciting and ever-improving end-game and probably routine level increases.
What I'm going to propose is a sort of "Best of both worlds" approach, which rewards each style of gameplay without making both mandatory. We further need to ensure that we not stratify the game badly, thus dividing the player-base.
The game is divided into two sections: the "Class" section and the "Legend" section. There are 18 class-specific levels(where you receive special perks based on your level) and 4 Legend levels where you have access to the same legendary perks as everyone else. The legend levels will call for dramatically more experience points to advance, but offer compensatory increases in power and ability.
At 18 and above, characters will have the option to "reincarnate", drop themselves back to level 1 and chose a new race and starting class. The second life will require 30% more experience than the first, and the third an additional 30%. However, starting with the 4th life, XP requirement goes down for each subsequent life by 10%, to a minimum of 80% of the original first life.
The rewards for Reincarnation are mild. If you had at least 10 levels in a specific class, you get to chose between a special 'perk' with a minor stacking increase related to that class, or an extra stat point. If you had fewer than 10 levels, you just get the extra stat point. More/better rewards can be considered.
In order to Reincarnate, you will have to obtain a special item. This is bought with Favors from the Highers, and since favors can buy other things and there are limits to how many you can earn in a day, this serves to "gate" reincarnation.