I read somewhere that there is one insight Einstein had (no it was not e=mc squared) that others in time are not likely to have fathomed, and one thing Newton did, that others would not have grasped onto either, for a very long time indeed. What are those? I don't remember!
My guess on Einstein is Relativity. That was pretty weird for the time (and still is), but I am sure it would have been figured out as we found more evidence that fast moving particles age more slowly.
There are particles (mesons) formed high in the atmosphere due to collisions that should only be able to travel down so far because they have a short lifetime. But they were found at a lower altitude, meaning they aged more slowly in our frame of reference. This one would have been solved. It was just too odd to ignore. Also, there were astronomical anomalies that got people thinking, too.
As for Newton, all of his stuff would have been done eventually. But what made his work so good was the fact that physics was pretty bare at the time and he didn't have a lot to work with in 1666 at age 24.