Part of the appeal of the game is the aspect of experience, capability and legend. Without these elements there is nothing special about being a level 50 wizard for example. Wizards who have been at level 50 for a long time have no advantages or respect versus someone who achieved level 50 just days ago. Again to quote Syndrome from the Incredibles movie: "when everyone is special, no one will be."
To address this and the time it takes to prepare a new world etc, I have a proposal that I think would be reasonable to level 50 wizards and achievable from a technical standpoint on the KingIsle development side. Simply create a SQL statement that calculates the time between achieving level 50 in the game and now (using one of the timestamp fields in the game database), split the difference across some ratio of training points and award wizards that many XP. New spells and could come before or after the release of a new world (like the stun shield distro) and I think at least level 50+ would be happy simply to be recognized as long timers whether or not the new world was out with new quests.
By doing this a long-time level 50 wizard would have at least the advantage of respect while vying side-by-side for quest-experience-points in a newly released world. Another minor placation would be the addition of a badge indicating that the wizard was of a higher status level-wise.
While I know you have excellent people on staff already to work on these kinds of things I would not want to miss the chance to humbly offer my free assistance working with your people on developing this kind of update as it's what I do on a daily basis IRL using the same dev environments and technologies.