Okay, I'll bite....why Diego?
It’s within range, assuming the plane had the minimum amount of fuel. But it’s a tiny island — filled with and surrounded by radar and communications — and there’s no way a plane could land there undetected. If you want to believe the US hijacked the plane, go ahead, but please remember to put on your tinfoil hat...
If you extrapolate the flight path after the change of direction (if the reports about that are correct) it continues on to Diego Garcia. Obviously, a US base would be an attractive target to muzzie terrorists.
However, I think India is more likely as a target, as a crash in the middle of Mumbai or a similar city would be more along the lines of the effect the terrorists would want.
I think it is very unlikely their plan was to fly it to a hidden airfield somewhere and use the plane later. Too much risk of detection before they could use the plane for a later mission.
Most likely the plane crashed into the Indian Ocean.