Interesting thread here, and it's good to see more and more consideration regarding MMA. My fear is that we have already identified the platform; some interesting stuff in the August Air Forces Monthly on Sentinel as a maritime platform.
A MMA must be capable of ASW, ASuW inc MCT, SAR, MTI and have an ELINT capability of sorts. It must have range, speed and an ability to carry ample bouys, whilst also being able to Process and Disseminate vast amounts of information in a timely manner. Talk of smaller interim choices merely to fulfil the ASW role would be a mistake. The UK must wake up to this - it's time to move on from ageing fleets with limited potential......there's no reason why said MMA could not fulfil the role of the E-3D also. Undoubtedly, lack of money will be the stopper but replacing 2-3 fleets of aircraft which require significant overhauls, with a more stream-lined and flexible mission system must be the way forward.
The threat from submarines is as strong as ever.......I would argue that it's increasing with the wider proliferation of AIP technology.