I've had this idea for a while, since at least 3.0. So one of the things that has always bothered me about level advancement and such in DnD is the sudden jump in power. All of a sudden, literally from the one hour to the next, you suddenly are more durable, cast more potent spells, are stronger, faster, and hit more accurately with your weapons. Granted, that is a feature of a level progression system. GURPS flows a lot more "realistically" with a slower, steadier progression based on your actions.
One of the original ideas I had in this vein was that when you level, you can only improve stuff you used. You can't add skill points to Listen or Jump or that sort of thing unless you actually use those skills at some point during the past few scenarios. You can't pick up Dodge unless an attack misses you or Mobility unless an attack of opportunity misses you while you ramble on through a battle mat.
A later idea I had in this vein was that instead of experience points improving your level, you spend them to buy feats and skill points and spells and bonuses to saving throws and attack rolls and that sort of thing. With this system, you're more versatile and can basically make your own classes and advancement. You don't have to take levels in Wizard and Fighter to be a fighter mage, you can just focus on purchasing attack bonus, weapon proficiencies, Intelligence and Strength bonuses, some spells and that sort of thing. Unfortunately, I could not mathematically come up with a system to break down the point costs of various abilities and the potential cost for them. I tried, oh did I try, but it was a trying system to implement. Plus, most of my players don't particularly care about how leveling happens, just that it does eventually happen.
One of the things that brings this old idea to mind is playing Skyrim, or watching Tony play it. It is a pretty simple system, the more you do stuff, the better you get at it. Shoot bows and you get better at bows. When you improve a set number of skills, your level increases and you can increase health, stamina, or magicka, and you get a perk. Perks are perks, they are like feats that tweak something. For instance, an archery one allows you to zoom, and a light armor one makes it so light armor doesn't weigh anything when you have it equipped. It is a system I've always loved, and one that kind of resembles some of the down time skill improving systems present in GURPS.
I'm not too overly concerned with this alternate leveling system, sudden increases in power are kind of a sacred cow in DnD. It is after all a class based system and not a skill based. It was just kind of a neat idea that appealed to me and Skyrim has it on my mind. Also, Skyrim is a pretty cool game.