Those of you who are familiar with CRPG (Cromedome is the most often host of said game mode) will find this fairly amusing. There's also a flash game (I think it's actually run by unity) that has this exact idea but I can't seem to find it at the moment, I will definitely link it later if I find it.
THE CONCEPT:
As you "play" you level.
This sounds really basic but hold on an lemme explain. As you spawn for the first time, you are given a notice: "this server is running leveler! Please be aware and if you need any help bla bla bla."
As they spawn, they are shown a list of levels, specifically their own: running speed, crouch speed, health, focus (ability to aim with right click weapons), stamina, jump height, hip-fire accuracy, etc.
Example:
AdinX spawns in the server, and all of his levels are 1
He walks forward, and his running speed reaches level 2 immediately. Now instead of running at .5 bricks per second (or something I don't know.), he can travel .62 bricks per second.
Then he jumps onto a plate and reaches level 2 jump height, allowing him to jump atop 2 plates, then a brick, then a 2x height brick, a 3x height, etc
But all of a sudden AdinX finds a sword, and an enemy
He runs up to the enemy and slashes at him, dealing 5 dmg out of a possible 50, but his slashing skill leveled so now he deals 8 damage per swing.
The monster hits back, and AdinX loses 5 HP, but his HP levels up so now his max HP is 15 instead of 10
This topic has been posted in the hopes that I can get some feedback, I know these types of mods are difficult to get people to like because at times they can create unfair advantages (lvl 50 player vs lvl 5 player, obviously the 50 wins), but I personally think this has a little bit of potential.
TL;DR-
Take the sims leveling system (level 6 cooking, level 2 music, level 5 speech, etc) and use that for basic stuff like how high you jump, how accurate you are, how fast you can sprint, etc