First.... I can't take much more of Wolf Blitzer. He's gotta go. Painful to listen to his "expertise".
Second.... How about an out of control avionics or electrical fire at altitude? Improper maintenance or something leads to a fire that takes out an essential bus that has the primary radios and transponder on it (or they quickly start pulling circuit breakers when smoke or flames enter cockpit), they make a turn to try to divert but are overcome by smoke or fire which takes out passengers as well. The San Fran crash shows the 777 can take a lot of damage but stay together for a while (perhaps while on fire). Fire burns itself out as they climb or run out of oxygen (or maybe it just keeps smoldering/burning) but plane keeps flying off autopilot and makes the altitude changes shown as it flounders around until it just runs out if gas or belly lands intact into the ocean and eventually sinks with no ELT going off.
Thoughts? Just an option that doesn't involve suicide,terrorism, aliens, or another season of Lost.