Every opening partnership needs one hit and miss guy, who will either give you a amazing knock or get out early, and the other batsman needs to consistently give you a good start. We had this in SOuth Africa for a while, with Gibbs and Smith. Gibbs was a very hit and miss player, and on his day he was simply destructive. Smith was much more consistent, and would almost always provide some sort of start and base for the middle and lower order. Australia also had this with Gilchrist and Hayden. Hayden was a but more consistent, but both were class players in their own right., Australia just need somebody there with Watson who can provide consistently, so Watson can be his free-flowing self. Marsh seems like the best bet from what I have seen of him. Hell even Katich could be a choice, but I don't think they will re-call him into the ODI side.