Unbuckled passengers injuring other passengers or flight crew during incidents like these is my second greatest concern (after airborne infectious disease) when flying commercial. I cannot imagine what it is like to have a 220 lb seatmate land on you during turbulence.
Not sure what can be done other than the preflight announcement to keep belt on when seated. The belt does not have to be tight just on enough to keep you in your seat. Perhaps that could be encouraged by flight attendants walking around the cabin since their primary duty is passenger safety.
Is seatbelt on when seated an FAA requirement? If not maybe it should be sort of like not blocking the aisles.