Some games did that(in part).

The original Gothic for example. When you were high level, other low level NPC would rather run.

I also thought hard about the concept, how to make FPS games still fun, but a bit more realistic. The thing is, in most settings this means reducing lots of enemies - as realistic would be, once you start shooting, they all come for you. Not 3. And then maybe another 2. And so on.. and then you would not have a chance, unless you get special powers (or quick save and quick load part of the mechanics)