[JKT]

Doesn't OmniDazzle's Focal Point already do this? Note, you can toggle how much focus it gives an app - so it won't focus on input areas for example unless you want it to. The only thing it won't do is let you use menus without changing the focus away from the app window (but that makes sense anyway).

The one usability flaw with it is that it doesn't shift focus to drawers or palette windows when you use them.