I have been looking through a few abilities that could be classified as high risk high reward without them being too overpowered.
Flare Boost is obviously a good mention since it boosts special attack at the cost of being burned and lets people take risks by switching out of pokemon an opponent would like to burn and letting it burn this to make it more powerful.
Analytic is a good choice as well making you sacrifice speed (a big risk in any game) to hit more powerfully and this is another high risk high reward style of play.
Sniper is a less mentioned choice but can pretty much define what we are trying to do here. If we would give this pokemon high critting moves like Psycho Cut, Night Slash it could make this type of play style a lot of fun as long as we don't make it able to crit all the time.
Wonder Skin is the last ability I want to mentions right here because it revolves a huge risk in using it as it lets you sponge status moves half the time instead of all the time like Magic Bounce and makes you take a huge risk in using it. Itself it isn't that strong of an ability but I think its still worth mentioning.
Well comments would be appreciated.
EDIT: Forgot to add Trace as an ability since it is another risky ability but its pretty good as well. Other people have mentioned it before but I support it as an ability too.