Game Idea: To get stronger, you have to take the power of you allies (power transfer)
The mechanics are simple. For your main character to get stronger, e.g. better stats, higher level, more weapons or abilities, your allies in combat have to get weaker. You start with a level 1 sack of potatoes with little ability of his own but is accompanied by one or two bad ass no-McGuffin types. As you play through the game, the player gets chances to steal the powers of his allies to make himself stronger. You can balance this out by requiring the use of each character throughout the game, so you have choice of power-transfer but you make parts of the game harder for yourself.