See Guild Wars for an example on how to do this properly. More time with the game should open up different ability combinations, not more power.
Properly constructed sidegrades are better than linear upgrades. In TF2 the unlockable weapons are not required and mostly support certain play styles. If I had to start over in TF2, I wouldn't mind all that much; there isn't any progression test I need to undergo to prove to Valve that I know what I'm doing. It's all player skill.