Ok this might be a little silly but what if xp was something to be feared? People gain XP at a higher and higher rate even if they don’t turn up to games. If one is a mage all that it can be spent on is more magic. Power is gained at an increasing rate until a critical mass is reached. Once you pass that level you explode or something else nasty with lasting effects in the game.
But even before that if there is too big a build up in the area they will come for you. Who they are and what they do with you is unknown to the players. None mages gain xp as well but at a slower rate. At any point they can awaken their magic. ALL there xp is then converted to magic but reguardless of how much this gives them they’ll be allowed at least one spell before they explode or whatever we come up with.
This could be happening to the entire world. Magic is growing out of control and none ofthe players know why.
It probably needs a lot of work but the idea being power is to be feared both by those that have it and those around them. No matter how much good the mages do they’ll be feared because each time one of them reaches the limit they leave what seem to be permanent changes in the world. Out of control effects. While once in a while there might be a good one (making for interesting locations) most are just bad in one form or another.
Even if you don’t show at an event people gain xp at a fairly standard rate. Bonus’s potentialy available for coming could be more control on what to spend xp on or even leads on ways to lose power rather than gain it. If you’re not a mage you have nothing to spend xp on other than magic and the temptation will grow stronger over time as more and more power is being ignored.