It would be very interesting if we reach a point where Protoshares can be traded over the Mastercoin decentralized exchange, or Mastercoins can be traded over the Bitshares decentralized exchange. We will get to see the true market value for each.
To do it in Mastercoin someone would have to create a virtual Bitshares backed by Mastercoin in escrow. At least that would be one way to do it and people would send some cryptocurrency to the issuer of virtual Bitshares and these could be traded as if they are Bitshares on the Mastercoin protocol.
To do it in Bitshares my understanding of it is that a cross chain trade would take place and you replace the example in the whitepaper of bitBTC with bitMSC. Basically users could trade bitMSC for real MSC 1:1 allowing MSC to trade on the Bitshares network. This could potentially improve the stability of both as individuals could hedge between the two.
I'd like to see cross protocol reputation. Meaning if someone builds a reputation on their Keyhotee would it be transferred over to the Mastercoin protocol so that the reputation functionality of Keyhotee can benefit the Mastercoin eco-system? If that is the case then I see Mastercoin and Bitshares having a great synergy because they can solve each others problems.