Why go to such trouble? Possible explanations
Consensus is it was a deliberate act with significant knowledge and non-trivial planning and flight for several hours either on Northern arc through China towards Eastern Europe or South to Indian Ocean. No single convincing explanation has yet emerged as to why go to these lengths. A few thoughts.
A) The theory of going North and landing somewhere remote for re-use as a weapon is interesting but I am assuming all strips of sufficient length are known as a result of a Cold War spanning many decades after the advent of high resolution satellite imagery and have been investigated by now by satellite, and ones with convenient hangarage more directly (although we may not know). One would hope that (at least) until it is found all the military around will be on high readiness and alert for any unexpected primary contact or questionable secondary and a T7 trying to creep along slowly at low level pretending to be a GA VFR flight with no tx is likely to get noticed, so 2nd part of plan seems less likely to work, we hope. But that's not to say any terrorist who has done what seems like the difficult bit was sufficiently rational and self-critical to have figured that out, and may have a plan. So I'd say this is still possible but not that likely.
B)Or did they carefully plan everything else but then have a "Doh!" moment by overestimating fuel on board or burning more than planned in initial high climb then lower level flight and fail to make a visible target perhaps in Europe and crash somewhere dark in remote Eastern Europe, without a fireball after engines flameout, still waiting to be discovered. Seems possible, question is could they have disabled ELTs first (and would they have bothered if intending to hit somewhere else anyway).
C)Indian Ocean if you want to make it hard to be found, this could be the pilot wanting it to be a long time to have cause discovered so his estranged family don't suffer, but arguably a dive at Vne into ground in remote jungle with lots of fuel still on board possibly with some erratic inputs, less planned and muffled struggle sounds on CVR (and maybe even a radio call suggesting distress) in case it survives would seem simpler unless he really wanted to keep a mystery alive. So possible but frankly seems unlikely. I would remain sceptical of any conjecture based on his t-shirts, family situation, etc as I suspect a sizeable proportion of professional pilots who won't ever plan or execute a complex disappearance have personal lives which could be construed as indicative of latent dissatisfaction with something. Now if his flight sim happens to have a route corresponding to route flown and relevant recent manual extracts, then we might reconsider but until then nothing to me supports this.
D)Or someone else who actively wants extended mystery, perhaps a true Bond villain type organisation who wants to show it can make a plane disappear... yet then apparently keeps quiet? Of course they may be in secret discussions as to what they want with some vague threat to repeat if motive is financial and they seriously think (unlikely) they can elude most of the world's co-ordinated efforts to track them. Seems unlikely.
E)Personally I doubt whether a religious or political fanatical terrorist organisation would go to such systematic lengths and not want a show either with a dramatic climax or publicly claiming responsibility and I suspect all chatter from all known such organisations has been minutely scrutinised for any hint. Time may reveal more, but if something useful were known or even suspected, there would be a lot of pressure to reveal at least something by now to allay public unease that authorities didn't have a clue.
F)Or a true "terrorist" wanting extended uncertainty - I suspect many rational pilots and passengers are more uneasy about flying and this may give a similar knock to aviation as 9/11. Perhaps a fanatical environmental group unheard of previously, many environmental activists are intelligent people with the wit I guess to access information, and off the radar (no pun intended) of conventionally blinkered security services. A recent BBC Horizon as it happened showed a group of trained US security people all failed to spot a similar threat and only a newbie did as the experienced analysts all looked for the usual suspects. Worryingly it seems hard to rule this out, though I suspect if it were correct, it is unlikely to be repeated, which if appreciated makes it less worrying, I suppose! Possible but also seems unlikely.
G) Interestingly the northern track plus any flying beyond last "ping" gives a good chance of aircraft being in China. Now given that China was the original destination, and the passengers were naturally full of people who know the country, it is not impossible that there was a plan to throw people off the scent then head back to a point in China for some purpose. Now just suppose (admittedly no evidence I have seen to support this) Chinese military discovered it some time after it had first entered Chinese airspace then hastily intercepted and shot it down. Would they A) freely admit to the watching world their air defences are porous and that they have just shot down an aircraft with hundreds of their own citizens on board or B) keep quiet and go on the offensive and criticise another county's incompetence for the incident? I don't know and it would be speculation from me or anyone else without inside knowledge to pursue this thought, but I can't quite see yet why this can be ruled out.
Before anyone shouts (although it seems this may be summarily deleted) I fully accept there is no evidence (to my knowledge) convincingly supporting or disproving any of these theories yet, which is my point. It would be good if at least some of them could be concisely sensibly rejected with even a single hard fact, rather than just someone else's supposition that they are unlikely. I personally don't see a better explanation out there yet, though there may be one. So applying Sherlock's principle, one of these unlikely theories for an unlikely event may be correct. I fear however, hoping to be proved wrong, that the only fact which will do the trick is the finding of the aircraft, which may take some time.