Well, players with extremely powerful abilities might just say they haven't come to fully understand or control their powers yet. Having partial or uncontrollable abilities could make something overpowered a bit more fun.
Also, setting-wise, I think the hunger games-esque idea would be cool. Or something more X-men like a powerful evil power user is mobilizing as many power-users as possible (some unsuspecting, some hungry for conflict), while a small collection of good power-users are trying to stop them (either commissioned by the government or as neutral parties). Something else I was thinking about is maybe monsters or god-entities are threatening the planet, but their presence grants regular humans powers, and so the power-users have to decide which side they want to be on.
And power origins could come from any number of methods. Injections, natural occurring powers (often surfacing at puberty), an event causes powers to randomly appear in people throughout the world, interference from other dimensions/planes (maybe even a god-plane or something).
Anyway, that's what I got.