AH, RIGHT.
Okay, so, very broadstrokes, one of the basic and most prevalent problems with XP in MUing is that you get DINOSAURS who've been around for a long long time and then you combine that with people who are active and create a horrible MECHADINO who not only has XP for being around so long, they are super active and continue to rake it in at a disturbing pace.
Not only that, you have people whose playstyles vary due to preference and lifestyle--people who can play for a month or two very intensely and then be almost completely absent for another month or three because of work, life, burn out, whatever.
So what I propose is an ever rising ceiling to the XP that can be earned in TOTAL. This ceiling rises daily by a very small amount (or very big, whatever the fuck you as the game runner wants).
So let's say on March 30 the XP Ceiling (XPC) is 40 XP (abstract number, I don't know how XP works in EP). The absolute HIGHEST amount of XP ANYONE can have on the game is 40. Let's say the ceiling grows at the rate of 0.5 XP per day. On March 31 it would be 40.5, and on april 1 it would be 50--JK, 41.
Then you simply allow people to rake up XP in several different ways. Let's say you put a +vote system with a limit of how many +votes you can give and receive; then you also give out weekly XP (I suggest this weekly XP actually be daily, since the XPC grows daily--I also suggest that the amount be high for new characters and taper off to something negligible, amounts based entirely on how far away you are from the XPC), and then you give them the ability to get a ton of XP (depending on whatever factors you wanna set) based on running and participating in plots.
What this creates is a system qwhere someone new but very active can actually eventually catch up to a dino, even if the dino has been around a lot longer, because the dino can't advance at the same pace anymore.
It also gives people the ability to play intensely for a few months, (perhaps reaching the XPC), but then go inactive for a while, only to come back and have a much higher XPC, that they can put their energy into reaching via plots, etc.
It's important to have many avenues for acquiring XP instead of just one or two, and have each avenue encourage certain type of RP. +votes probably works for day-to-day social RP, but I wouldn't allow people who are in a plot to be able to +vote for people just because they're in a plot together, etc.
The rate at which the XPC rises should contemplate the rate at which you want the PCs to get more powerful at the ABSOLUTE FASTEST.
If you don't want them to have "more than 500 XP at year's end" then the XPC needs to rise at a rate that won't allow them to have more than 500 XP at year's end.
Make sense?