Personally, I think it is most likely terrorism. The smoke theory is a good one though, and in that category I would lean more toward avionics. A tire, especially a nose tire (because they rarely catch fire), is unlikely to cause flight deck asphyxiation.
Here’s why i’m not buying terrorism. The whole point of terrorism is to terrorize... not just the people on the plane, but everyone else. There is an agenda, or a point to be made, no matter how misguided. But if you just hijack an airplane and fly it into the ocean with no one else knowing, its not terrorism. It’s suicide and murder.