Since we're talking about high risk high return, what about Hustle? I mean, Hustle is an ability that sacrifices Accuracy for Attack. So the attacks have a higher chance of missing but when they do hit, they deal more damage. For the Hidden Ability, I would think Guts, Flare Boost or Toxic Boost is a good idea. Or maybe Reckless.
And since we're on to new custom abilities, why not something that is similar to a Life Orb? The user will have its attack boosted by X% but will receive recoil of Y% of its health.