Well you pointed out one biggie: the known laws of physics. Anti-gravity may be possible, but there is no theory to explain it, and so it’s hard to predict in a positive way. By contrast advances in materials science, miniaturization, novel sources of energy, are more fruitful.
It can also help to look at the state of current laboratory research. Look at where transistors were a ~100 years ago, and then now. Look at materials with great promise on the lab, but which may take similar timescales to become commonplace. It’s not the new discoveries, but the new means of bringing old discoveries to mass market which often defines tech.
Transistors weren't anywhere 100 years ago. The first practical, working transistor was in 1947. The theoretical foundation (quantum mechanics) wasn't particularly solid until the 1920s. So to get from where we were 100 years ago to where we are today, there were both changes in the known laws of physics, and also practical advances.
But then the question becomes 'literally anti-gravity', or more likely, 'personal flying devices'. Certainly not the same thing but, for the most part, the same effect.
When it comes to flying devices for the individual I think the thing we learned about all is just how little we want people raining down on us from the sky. Not to mention the difficulties of air traffic control with a few hundred million people in the air. So ask yourself if FlyingtechX can really solve that?
My vote is on quantum levitation. Wings, jetpacks and mini-helicopters are all already hitting the limitations of their physics and still far away from anything you would casually carry around.
It can also help to look at the state of current laboratory research. Look at where transistors were a ~100 years ago, and then now. Look at materials with great promise on the lab, but which may take similar timescales to become commonplace. It’s not the new discoveries, but the new means of bringing old discoveries to mass market which often defines tech.