I like your idea for simulator-flight-completion-time vs. supposed flight speed.
Just two days ago I flew into Madras, OR for the eclipse. Me and 400 of my closest friends. Per the NOTAM, at a certain checkpoint we were to be at a target altitude and speed, and maintain that speed into the pattern. For my aircraft, I had to slow down from my economy-mode 140 mph (I coulda been doing 165-180), down to 100 mph, and I can't do that (and also lose a lot of alititude) at the drop of a hat -- I need some lead time.
But regardless of that very specific instance, every time I fly into an airport, esp. one I'm not familiar with, I'd like to be able to set a waypoint(s) prior to the airport with an assigned speed for each, and have the software start warning me to begin slowing.
Or maybe at least have the ability to set a popup message for a waypoint. So even if we can't set an assigned speed for that waypoint, we could at least create a note to ourselves that popups there. That would be very general purpose -- we can remind ourselves whatever it is we need to be doing at that point.