Hi Buster,
Sounds like you have lots of good ideas, although I'm afraid you may find that the big GPS manufacturers may have beaten you to many of them. Check out
Garmin's website, particularly the small handhelds like the Pilot III and the GPSMAP 196 - these already feature an incredible array of 'situational awareness' features including warnings on approaching controlled airspace etc, calculating ETAs to each waypoint, fuel remaining, etc.
However, as Genghis says, GPS is no good for the other parts of your plan, to do with warning the pilot of an impending stall/spin situation. Predicting an impending stall is *extremely* dependent on the local wind conditions - and I don't just mean headwind/tailwind. Much more important and dangerous are small scale changes in wind conditions - updrafts, downdrafts, wind shear, turbulence, etc - and GPS can't tell you anything about those, unfortunately.
To my mind, a device that would be useful, both for stall warning and more generally, would be an angle of attack guage. As you can read in
John S Denker's online book, an awful lot of key aircraft performance configurations (stall, best glide) relate to a specific AoA, but we pilots have to try to achieve those configurations based on indicated airspeed which is only indirectly related to AoA... An AoA guage, perhaps with a predictive warning based on rate of increase of AoA, would be an interesting and useful device...
cbl.