If the plane didn't crash somewhere in the Indian Ocean but was in fact successfully commandeered by the forces of darkness, here's what's going to happen:
A lick of paint and a communications makeover sufficient for fooling the world for an hour or two. One day, an already targeted 777 will be incapacitated midflight and immiditately impersonated by the refurbished MH370, now with unhindered access to the airspace of whatever destination.
On board could be anything heavy and unwieldy - a small army or a large, primitive thermonuclear device.