The thing I always toy with, game mechanic-wise, is how odd it is that when players get more skilled they are rewarded with powerups. They've already proven they can accomplish a task, it seems counter-intuitive to then make them better.
Yes, I get that this is offset by giving them more difficult opponents or a wider breadth of options, but then you just get into an arms race which ultimately gets kind of foolish. It seems games seldom make use of scarcity, or making abilities more limited as a game goes. You start with advantages of weapons, regenerating health / armor, all the things which make you 'more than' human. And then on subsequent levels you lose those advantages.
What if you start as some flavor of 'Speece Marine', then after doing something appropriately heroic your next task is (loosely) the same thing with some average grunt. And if you succeed there, the next iteration is with a civilian. And then an elderly civilian or a kid or something. Because, look - if having Master Chief mow down bug eyed monsters is mildly impressive, seeing granny hose down the Alien Queen and attendant hoard is simply jaw-dropping.
Plus think of how fun this mechanic would make a CoD-ish spunk-gargle-weewee game. Your n00bs, the first time players, they would take to the field with airstrikes at their beck and call. But as their experience grew, their weapon choices would become more limited. Their magic regenerating armor / health would stop. They could be one-shotted. Weapons would do less damage, requiring more teamwork. Become slower and lose the ability to jump meaning you have to re-think how you navigate maps.
Over a game's life-cycle, as you have fewer and fewer people joining, you have fewer and fewer 'hero' troops and more masses of regulars. Which seems... well, something which would help define 'hero'. The canny oldsters would be the veterans who can stand on their experience, not their perks.