How's this for speculation: hijackers took over the aircraft. Pilots cooperated in turning off transmissions. Situation deteriorated, and instead of losing control of aircraft, the hijackers released a nerve agent that killed everyone and the plane continued on its last heading until it ran out of gas.
I'm not smiling, but I've been thinking why fly out into the Indian Ocean and it makes no sense, unless someone wants to hide something. What would that be? Which brings me back to one of the original theories, pilot suicide and he wanted to spare his family from shame, by trying to dump it somewhere nobody would locate it and figure out what happened. In that light it's really maddening that someone would have no compunction of taking an entire aircraft of innocent people with him.
That's more probable than a lot of the other junk floating around. IF what they have reported on TV was the case (re: location) then something along those lines is probably what happened.
No matter what though, with the amount of satellites that are constantly surveying the Earth, it doesn't make sense that they lost the aircraft. The technology Google used to take a picture of the car on my driveway (that is available to the entire world) is over 10 years old. The lenses they use now with tracking sensors are super advanced. Also the drone deployment capabilities we have today are insane. They can deploy a network of drones to literally any location within an hour. So if the plane was aloft for as long as they say, there's no way it goes missing.