A futures contract is an agreement to execute a trade at a future date at a previously agreed price.
For example, say Alice agreed with Bob to pay 1 bitcoin for 6 barrels of oil in September. Sometimes, these can be cash-settled instead of actually delivering the oil. This is useful in hedging exposure to the oil price. In this case, Alice and Bob need to agree an impartial, external source for the price of oil in bitcoin on that date.
Thus, most discussion of decentralised future or forward contracts in bitcoin land centres around the use of an external 'oracle' to provide such a source. This introduces a third party and goes against the trustless spirit of bitcoin.
The future difficulty is of great importance in the calculation of mining profitability, but it is difficult to predict. So, miners badly need the ability to hedge or accurately price difficulty.
My idea is that, in the special case of difficulty, the blockchain is the authoritative oracle, and an external oracle is not needed. Thus, trustless decentralised difficulty futures contracts are possible.
Does anyone have any ideas how this could be implemented?
(x-posted reddit
http://www.reddit.com/r/Bitcoin/comments/2a5jrr/idea_mining_difficulty_futures_contracts/ )
What you are describing isn't a futures contract, but rather a form of gambling. The first example you gave, where I agree to trade you 6 barrels of oil on September first for 1 bitcoin, is a futures contract. Assuming we don't choose to exchange cash instead, you will get your 6 barrels of oil and I will get my one bitcoin.
The second example, what difficulty will be on September first, isn't a contract at all, nothing is being exchanged. It is simply a bet on a future life event. Any gambling house would be well equipped to lay odds and accept wagers on the event.