I wish that instead of constantly increasing the level cap, MMORPGs would allow more opportunities for the more hardcore players to improve their characters that would also help improve the game for more casual players instead of getting people further spread out in level and gear.
One such mechanic that might help avoid that would be a rebirth mechanic that let the most powerful characters restart the game from level one of a different class. To make it interesting, it could be implemented such that you could keep one skill from any of your classes while playing another class. For example, if such a system were implemented in EverQuest II, perhaps after a player got to level 80 as a Guardian they could restart as a Monk, level that to 80 as well, and then perhaps while playing as a Guardian they would be allowed to use any one (and only one) Monk skill (such as Peel or Tsunami.) If they are reborn multiple times, they will have more alternate class skills to choose from to augment whatever character they play.
This shouldn't make it so unbalancing that hardcore players get too far ahead of the rest of the server, and it might actually improve the game for casual players, since it would get more people to go back through the lower-level content. While leveling an alternate class, the XP gain might also be slowed (since the character is technically still level 80, he wouldn't gain Vitality, and perhaps XP gain from quests and adventuring might be slowed even further.)
This would allow old content to be stay relevant in an aging game that doesn't attract as many new players. It also means that a lot of quest items (such as heritage quest rewards) have value even if you aren't currently playing a class that the reward is appropriate for. Because players would need new quest lines to help them advance (since they will most likely have done most of the existing quests already), the developers would have an incentive to revamp and add to old content (which could also help attract new players), instead of simply adding tier after tier to the end game.