Have you looked into Ozcoin's PoT payout system? It rewards each share differently according to the minimum difficulty you're looking for and the difficulty of the share you submit.
It also completely adjustable, and by setting a really high work difficulty, you're increasing variance drastically. You won't get paid for as many shares, but the bigger shares pay off WAY better. It's essentially the gamble you're looking for.
Example: You set a "Work Difficulty" of 100,000,000. Most of the time you will not return shares that have a difficulty of 100,000,000 or more, so these don't get paid anything. When you do get a share above that value, you get paid on the value of the share.
In the example above, a share of 99,999,999 gets paid nothing, a share of 100,000,001 gets paid ~0.0032BTC, and a share of 50,000,000,000 gets paid ~5.14BTC.
Moving your work difficulty up from 100,000,000 to 1,000,000,000 moves your payout for the same 50,000,000,000 share from ~5.14 to ~9.61BTC.
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