Perhaps they flew into a time warp, and are now looking down at dinosaurs where Idlewild airport should be, but taking in all the latest... two passengers with stolen passports using cheap, one-way tickets purchased by an Iranian third party, I would put a hijack followed by suicide dive into the sea (like the EgyptAir flight) at the top of my probable cause list. A bomb at altitude would have spread floating debris for miles and miles, whereas diving straight into the water (or land) would leave the smallest debris footprint. Remember how long it took to find and recover debris from that ValuJet crash in the Everglades, and that was with ample radar tracking data over land. The lack of any distress call suggests either the sudden incapacitation or participation of those on the flight deck. If the aircraft was hijacked, perhaps with assistance of one or more of the crew, they could have pulled the breaker on the transponder and ADS-B, flown on to jungle of Viet Nam, dumping fuel along the way and then impaling themselves into the canopy with a similar residual crash site effect as lawn-darting themselves into the sea.
All we at the keyboards can do is speculate, but three things are for sure, 1) the aircraft is no longer airborne, 2) it will be found, and 3) we will be able to figure out what really happened.