There is so much fascinating stuff that we can't know yet because we don't have the technology to make systematic tests of it.
Like the question if personality changes when you wipe/replace memories. Does your personality change when parts of your brain are destroyed/rendered incapable? If that were true, it would revolutionize the penal system, for example. We don't know because we can't remove, replace or insert memories yet.
But if it turned out that memories do not have an effect on personality, well, shit. Then where would it come from? What if it turned out that recovered Alzheimer patients only lost their memories, but not their personality? We don't know that either because we don't have a cure for Alzheimers yet.
What exactly do people experience during death? We'd need to be able to record and analyze that. Also near death experiences. Also dreams. And coma. We don't know that because we cannot record experiences yet.
What would happen when you reanimate a braindead patient? Could he report anything? We do not know, we haven't been able to do so yet.
Could we find a way to communicate with comatose patients? That would revolutionize the way we treat patients on life support, and affect the decision when to pull the plug.
And who knows, with all that dark matter and dark energy shit, can we really say for certain that a soul/consciousness/sentience isn't something that we just cannot measure yet?
And the age old question. Does a dog have a soul? Does an ant have one? When is sentience sentience? At what point does brain activity result in awareness in the sense we experience every second? That answer would settle the "abortion is murder" debate.
If we knew what sentience was, we would know exactly what we'd have to do to replicate it. And then we'd be able to create truly sentient artificial intelligence.