A simple over/under straight bet. Based on your calculations, at the calculated date. If the difficulty is, say 98% or higher of your projected amount, you win. If lower than 98% or your projected difficulty, I win.
Five bitcoins is the wager.
Well as you can
see here I do not have 5.0 bitcoins : p (That is the address in my signature too, by the way).
I may only end up with 5 bitcoins just as the 2016th block is hashed in this current 877k difficulty round. Based on my calculations I'll only end up mining a grand total of 9 bit coins in the next 6 difficulties (55.8 days at the 9.3 days-per-difficulty at 50% increases from above). So wagering more than half my projected wealth seems a bit too steep for me : ( 15 days worth of hashing!
What projected date, though.
Block explorer lists the next difficulty at 1200518.48071190, which is a 36% increase. To be 50%, it would need to be 131k. Obviously the lower the earlier difficulties, the less likely it is that my 50% calculations hold up by the 6th difficulty round~