With this you can have practically 100% CPU for the foreground app, when everything else gets suspended automatically, making it essentially a single-tasking system, though you can exclude apps that need to run in the background. To me this seems a very flexible, universal tool, like WinWarden, ProcessGuard, and MouseActivate, obviously worth coding.
This is far from deep system engineering no matter how you look at it. Setting priorities breaks nothing, and that's been done in some scripts already. That's not what this is about. What I'm suggesting is building an invisible frame over every window (only needed in case you're going to suspend them) and sending suspend command to the associated window, and when hovering again over that rectangle (under which the frozen window remains) sending resume. That's all.
Another application would be auto-resizing windows, switching between two windows sizes when hovering/not. It's just passing commands to windows based on mouse position. The thing is, it's almost done already. I know nothing of this stuff so I would have to teach myself the language, inspect code, and hang around forums asking questions and finally, if anyone bothered answering, they would most likely end up writing the code, not me.