Possible scenario for deliberate hijack from pilots, and assuming that aircraft has landed somewhere: Attached is a computerised flight plan for a similar flight and very close by performance data for the same type. Pay attention that if the hijackers intended to safely land somewhere they should consider the landing weight and allowable landing distance. So at the point of lost comms the aircraft was still at the landing wight higher than maximum landing weight. You need to burn fuel (or dump fuel) to reduce the landing wight. With maximum allowed landing weight you would need at least 9000 feet long rwy. Then search for possible airfield to land should be focused on area that is at least 1000 NM away from the last known position in order to make a safer (below 460K lbs ) landing weight. Another factor a professional should consider is runway PCN - strength of the surface. Eliminating those factors the possible airfield should be limited to VGEG, VYHL to the North and Cocos Islands and Christmas Islands to the South. Other more realistic theory is that the airplane flew on autopilot (with crew incapacitated or killed) as long as fuel endurance allowed and then fell into Indian ocean. The aircraft would remember the last lateral track if navigated by the FMC, or set heading on the MCP.
My guess is that aircraft should be somewhere between Shri Lanka and Maldives.