@HelloProject There are so many ways you can do this it's not even funny.
For example consider horizontal rather than vertical progression paths. This is pretty common on many MMORPGs for example; on WoW the maximum level you can attain is 110, and XP get you there, but every 110-level character isn't at the same power level - they can get gear through doing dungeons, they can improve their artifact weapon through questing, and you should absolutely consider cosmetic improvements as separate progression paths as well because they are, ranging from achievements, titles, skins, mounts... players spend hundreds of hours on those things.
In a MU* you can do the same thing. XP could be absent or be tiered, allowing the fastest/most active players to get to the cap each time and then while they wait for everyone else to catch up you can give them different carrots to gain; reputation, ranks, in-game items, resources, land... then once the game begins to stagnate up the odds again and let the chase start anew. Or don't.
Game design is one of the most exciting parts of making a game, allowing you to pour a ton of creativity and cool new stuff into it. The only catch is once you commit to it you're sort of stuck, since converting to a new one after the MU* has launched and people are using it is a huge pain in the ass, so you want to hopefully figure this shit out before you implement it.
It baffles me some MU* runners redo what the last game did almost verbatim. It seems like such a waste of perfectly good fun on their behalf.