Null Levels
I've been kicking around a concept I call "Null Levels". I adapted it from some rules by a fellow named Aelrynth over on the Monte Cook boards.
If you desire, you can spend xp to gain a null level. This can be done once per character level. The cost is 400 xp times the number of null levels, e.g. 400 xp for the first, 800 xp for the second, etc. This is similar to the xp advancement for standard levels. Each null level gives you one feat, 4 + INT modifier skill points, and 2 + CON modifier hit points. There is no gain to BAB, Saves, or class abilities.
If desired, a character can use the feat to increase the uses per day of a class ability, but the character has to have levels in the appropriate class. You can't get or improve a rage ability unless you have levels in barbarian.
It is tracked similar to multiclassing, i.e. Human Paladin 3 / 2 null levels. You can't have more null levels than character levels.
To date, the players have made very sparing use of null levels, usually just to gain a feat to use an exotic weapon or magic item.