The P2P transaction embellishments seem to be coming thick and fast now. I have also posted an idea called 'bitstock', but it involves a separate network and blockchain, for not only contracts but a diversified stock exchange, payment gateway and P2P escrow. It avoids modifications to the blockchain, network or protocol. Existing clients would function as usual. Only users who wish to trade with duplex accountancy, would have to upgrade. No third parties either with this one.
You could still get a merchant bank account and trade in local currency for a nominal fee. Now you mention it, proper contracts with repeat payment schedule's a graphical form builder and client support database, would make a nice extension too. Better to build in functionality that needs adding than be left out in the cold.