1. Hardware Design - The physical hardware would look like a TomTom (perhaps ONE XL'ish), but would also incorporate the ruggedness of Magellan's Roadmate 2000/2200T/CrossoverGPS units. Oh, and it would be waterproof to IPX7 (could be completely submerged in water). The unit would also be light weight, so as not to vibrate when driving at highway speeds.
2. Mounting Hardware - The windshield mount would use a Garmin-style ball and socket mount, but would use higher grade rubber that doesn't melt to the windshield in the hot sun.
3. Display - The GPS would have a 4.3" wide, aircraft-grade super bright touch screen display
4. Traffic - Real-Time traffic is perhaps the most important feature of all, yet most companies treat it as an afterthought. The ability to intelligently navigate around traffic is an awesome capability that has a significant impact on the driver and on local municipalities. Given the importance of traffic awareness, I'd use two systems for the most complete coverage: TMC (Traffic Message Channel) from ClearChannel, and WiFi internet connectivity for grabbing live traffic conditions via Google Maps. The GPS would then use whichever traffic data is most current in the area you're driving in. Moreover, the GPS would support an open data format for exchanging real-time traffic information with other GPS devices (XML?), and would upload current traffic conditions so other connected drivers could download them (similar to Dash Navigation's solution, but would be available to all GPS users, not just Dash drivers)
5. WiFi - Speaking of WiFi, this imaginary GPS would be able to use its built-in WiFi for not only traffic data, but POI lookups, map updates, firmware updates, vehicle tracking via the web, movie listing, gas prices, and more.
6. Support - This GPS would automatically receive new, updated maps every 6 months. The fictitious company that makes this uber-GPS would continue to support it, releasing updated maps and firmware on time and for free.
7. Route Recording - This wunderGPS would allow me to record a specific way to get between two points (almost like a breadcrumb trail), and then use that same path on future trips. This would be a great feature if someone familiar with an area shows you a shortcut, or if you're following another car. This way, you could simply record the turns you made, and have the GPS know your preferred route (unless of course there's traffic or other problems along the route).
8. Learning GPS - Speaking of saving a particular route, the GPS would "learn" certain preferences about me. For example, if it detects that I always ignore the suggested route and take an alternate turn instead, it should learn that I prefer to go that way instead, and incorporate my own preferences into its routing logic.
9. AAA TourGuide - This GPS would incorporate AAA TourGuide data, as the Magellan Maestro units do, but would merge that data into a single POI database.
10. Zagat Restaurant Guide - The GPS would also have nation-wide Zagat restaurant guide information pre-loaded
11. Input/Output Jacks - The GPS would have a headphone jack and microphone input plug
12. FM Transmitter - The GPS would have a built-in FM Transmitter for piping the GPS' audio over your car's stereo system - like the Garmin nuvi 660/680, but it wouldn't suck (like Garmin's does)
13. Text-to-Speech - The GPS would use TomTom's Text-to-Speech (which is actually licensed via Nuance, the industry leader in text-to-speech technology) to speak actual street names.
14. Hot Parked Car Compatible - You'd be able to leave the GPS in the car without fear of cooking the LCD panel in hot sunlight
15. Improved Map Screen - The GPS map screen would be a mash-up between Garmin and Magellan's map page: it would use Garmin's overall map, but would include Magellan's feature of displaying an arrow in the lower left corner that displays the next turn. It would also allow you to toggle between displaying the arrival time, remaining drive time, and distance. This improved map screen would also display the GPS signal strength.
16. Routing Engine - The GPS would use Garmin's routing engine.
17. Route Exclusion / Detour - The GPS would use Magellan's route exclusion feature, but it would actually work.
18. Routing Methods - The GPS would include Magellan's four routing methods: shortest time, shortest distance, least use of freeway, and most use of freeway. It would also include the additional "user preference" route that would incorporate the learned driver preferences. More importantly, the GPS would display the estimated drive times next to each of the available routing methods so that you could actually know what the difference between the choices will be.
19. Global Maps - The GPS would ship pre-loaded with world-wide maps.
20. Fast and Responsive - This GPS should have a fast CPU so that ALL operations are zippy, including address input, route calculation, and map panning/zooming.
21. Longitude/Latitude Support - This GPS would be able to route to coordinates, and would support basic Geocaching features.
22. Multi-Stop Routing with Optimization - This GPS would support multi-destination routing with route optimization (the ability to sort a list of addresses by distance, so you are visiting each address in the most logical order and not driving back and forth).
23. HUD - The GPS would project the next turn onto the windshield, creating a Heads-Up-Display (HUD) so you could see the next turn without taking your eyes off the road.
24. Census Demographic Database - The GPS would include U.S. Census Bureau data that could be used both for exploring new areas (finding out interesting demographic information) as well as configuring routing preferences (avoid driving through high-crime areas).
25. Voice Recognition - The GPS would have voice recognition, like Magellan's Maestro 4050, but would support a full range of commands and would actually work when driving at highway speeds or listening to music in the car.
26. QuickSpell - The GPS would use Magellan's QuickSpell text-entry system, and would pre-populate address fields with recently used locations, as TomTom's GPS units do.
27. Exit POI with Split Screen - The GPS would have Magellan's excellent Exit POI feature (a great feature that allows you to see what POIs are located at each approaching highway exit), but would allow you to operate in split-screen mode, with the map shown on the left and the exit POI's shown as a scrolling list on the right side of the screen.

No doubt all of this wouldn't come cheap, so this would undoubtedly be a high-end GPS unit. On the other hand, it was only a few years ago that a basic GPS cost thousands of dollars, and less than a year ago a decent GPS cost almost a grand. So even if this fictitious GPS was costly at first, pricing would no doubt come down as demand grew.

Did I miss anything you'd like to see in a GPS?