Intro
This post is intended for FPSs but could be adapted for other genres and because I’ve only played a few games in one genre it may have already been proposed/used, in which case I apologize now. I’m not trying to steal the idea! Also, for me it’s just an idea to talk about and I have no intention of trying to build the system myself.
In general, the more you play a game the better you get at it.

Your aiming improves, the decision making process is refined, you become more aware of the types of traps that are set, how the shape of the level/map forces you movement in certain ways, in short you become more skilled. However, unlike the real world, your character never improves physically. No matter how much running or jumping you do he or she is limited to the limits set by the designer. What follows is a proposal for a system I call Improvable and Transferable Physical Attributes.
What I imagine in my head never seems to come out well in the written form, so please bear with me.

Improvable Fitness
I propose that the more you do a certain type of physical movement the better at it you get. You become fitter. Your fitness could be split into a number of facters: Speed, Endurance, Strength. By running quickly for short periods you speed improves. By carrying heavy weapons or punching/kcking/striking often you become stronger. There could be other physical attributes that designers could use like, swimming ability, ability to hold your breath underwater and flexibilty etc. But these would depend on the game in question.

Another feature not mentioned is the ability of players to create their own attributes but then the game would somehow have to use those and that may be impossible.

There could be specifically designed levels/maps that would improve certain aspects of fitness. Maps with steep hills or very long flat areas. If I play this map two or three times a week then my fitness improves, if I play this map everyday then my fitness will decrease, similar to the real world.

Transferable Attributes
Now, take the physical attributes and create an online profile. This profile would be used by any game that signed up to the system. Meaning if I get super fit in Half Life 2 I could import my settings into Painkiller (Games used as examples only). I transfer my other skills, shooting etc, why not my fitness?

Not exactly on the subject of attributes but continiung the theme of an online profile. Saved games could be included in the profile, meaning that players could continue a game from another computer without having to transfer data. Imagine a productive lunchtime gaming session at work and then being able to continue from that point on when you returned home!

Players would be able to create any number of profiles. Certain profiles would be better for certain games. For example strength may be a better attribute, in cramped-area games like Doom 3, than speed.

Difficulty Settings
The online profile would contain data that would help define difficulty settings within games. Instead of simply EASY, MEDIUM and HARD ememies could move at different speeds, have different strength levels etc. Currently difficulty levels seem to involve changing levels of fire power, health levels, ammo levels and the number of enemies per area. This system would enable designer to have finer control over the level difficulty. It may also mean the player could define exactly what areas of the gameplay he wanted to be harder, instead of everything.

Actual Physical Characteristics
This section is a little far fetched. Imagine going through a fitness test that let you imput your score into your profile, so that the character you play is almost you. Of course this would be optional, there are many people who play games to get away from their physical limitations. But there may be a hardcore group of CS players who want to add that extra level of realism.

Spoiling the game?
Of course there would need to be limits of how fit you can get otherwise players would become supermen.

Cheating
I’ve no doubt that the system would be open to hacking and cheating but my thoughts were more focused on the SP game and if players want to cheat when playing SP who cares? If the system was used for multiplayer games then it needs to be a very safe system otherwise it will be abused.

Conclusion
The idea is really split into two seperate idea; the online profile and the fitness of the player. Both are independent. It may be interesting to see if any players would take the time to build and players maps specifically to improve their game character.