Searching with the mag, even right now, really doesnt work when looking for an aircraft, a mag looks for differences in the Earths magnetic field, and there is not enough steel in the aircraft to provide a decent signature at depth.
Searches are concentrated on using multi-beam sonars to image and look for anomalies.
Improvemnts would be continuous broadcast/monitoring of aircraft in flight, longer ping time, dedicated ping per aircraft, better understanding of which frequencies to use for longer distance sensing, battery life, and perhaps an ejectible locator beacon...