Far be it for me to be suspicious, but state-sponsored terrorism seems at least possible.
Who would want a plane not directly attached to them? If you wanted to fill the plane with “stuff” and send it to someone you did not like, and the stuff was bad enough, no one would shoot it down over their own soil. And for another country to do so, it would almost be an act of war.
And, if the missing passengers were aboard and unharmed as precious cargo, it would also be messy to take it out. A plane full of passengers from a country that would lash out if the plane had to be shot down. A bad guy going anywhere he wanted could do so with a plane full of hostages, saying plausibly “oh, we just want safe passage…”
I find it interesting that the plane could make it to Pakistan. It looks like one could just about make it to Hyderabad, if one knew what he was doing and had the experience. I have included a shot from the link there…
Where from there, one wonders. Only a few states would want it, and almost any use you could think of (legit) would be easier done by just buying it outright. So let’s go to the shady uses – by means, motive, and opportunity.
Bad actors would be Pakistan, Iran, Saudi, Syria, and Yemen. All of whom could strike nearby by allowing their airspace to be used. The state sponsored hijacking would have to go through Pakistan because it is the nearest point.
The “end customer” could be anyone on that list. And many of those states have access to deep pockets. And the have access to bad substances, weaponizable. Means.
Every one of them have enemies. Israel and the US but other softer targets are nearby. Motivation abounds.
Opportunity… It would have to be an area with friendly soil nearby. It would be radar-sparse.
Loose conjecture, I know, but no crazier than stealing a 777 in the first place.