Emulators get really complicated when they have to somehow simulate events external to a circuit.
And to some extent, most debuggers are beyond the comprehension of the target audience. (Every debugger I've seen is essentially similar, and relies a lot on the person behind the screen having a reasonable knowledge of things like variable scope, stack frames, function linkage...)
But yeah, something like this would be nice, even if (especially if?) it managed to implement a simplified subset of debugging and simulation... (good research project for someone's Master's thesis?)