I suppose banks could create some sort of financial instrument that says "IOU 2.5 BTC" or whatever, but this thing they create couldn't pass off completely as b

I suppose banks could create some sort of financial instrument that says "IOU 2.5 BTC" or whatever, but this thing they create couldn't pass off completely as bitcoins, since you couldn't spend this thing on the blockchain like you can with ordinary bitcoins. Maybe this will happen. Who knows.