First, the pilot(s) could have just flown it up into the stratosphere and then straight down. Hoping the black box would be recovered to show they took it higher than anyone else.
Second, possible, is it was landed somewhere in Aceh at an Al Qaeda base. The passengers will have been taken hostage and spirited into the forest. Parts taken off the plane, perhaps some collateral passengers also, will be dropped into the Indian Ocean. Once this wreckage is detected the conclusion will be a crash in the deep Ocean. The search will commence over a large area for the body of the plane. It may take months to not find it, but even if they give up, then they cannot conclude it did not crash or entertain another possibility. When MH370 is recommissioned as a flying bomb it has enough fuel - being only a short distance from KL, in Aceh - to strike into South East Asia or the subcontinent without warning. Maybe target is other aircraft on ground, ship and not static target like a building. Could take out leaders at a world gathering within the strike radius. Danger already, but will relax guard as soon as wreckage is spotted. As for how a hijacking was done? Maybe a few female members involved in this action. Could have had two women in cabin with one pilot. Would account for last voice transmission.