This is just speculation, but given the absence of sightings or new leads, and with the information at hand, it will probably turn out that the pilot, the copilot, or another person having access to the flight deck and with a lot of knowledge of aircraft, took over the aircraft and ordered it to be flown to a point far SW of the last detected position, then crashed it into the ocean.
The possibility exists that we will never find the plane, because of the great number of possible crash sites and the absence of radar or other kinds of coverage in the South Indian Ocean region.
If the plane were flown to a land location and landed, it's very likely that someone would have revealed its whereabouts by now. A plane that size can't remain a secret for very long. This is why I think it was ditched in the water, and that implies that the plane and its passengers are lost.