I've had this idea for a while, but I'm probalby too lazy to implement it anyway. So I might as well put the idea out there for others to muse over.
Derivatives trading denominated in bitcoins. Futures, options, or entirely new types of contracts.
Obviously, we're not at the point where we can buy real stocks or bonds with BTC, but I don't see any reason why we cannot trade derivatives off of these instruments.
The only problem is that the market for BTC derivatives probably won't be very liquid, at least initially.