I think that developers should first choose conception they want:
1) "Vertical content implementing" = focus on implementing content for leveling chars
This means, that they can set higher lvl cap than 21, enable low speed leveling, but they will need to add much content into the game, because for every kind of characters, they will support, and every level, there should exist opportunity to entertain and level itself.
(lvl. 1, 5, 10, 15, 20, 25, 30, ... quests/enemies/weapons/...)
or
2) "Horizontal content implementing" = focus on implementing content for new roleplays
This means, that they will introduce a content (game play) for level 21 chars, content for lower levels would be only aid to get to lvl 21. Positive of this is that they will know how to balance the gameplay because, they will now what kind of best lvl 21 chars you can create, they can concentrate to add content to lvl 21 (and balance it). Problem is, that you can be bored after some time. But solution is to add content/roleplay for another chars.
Of course, that they can combine 1 and 2.. But it is very hard and time consuming work and i dont think this will work..