If you are looking for a software System Level Spec...my "wet-dream" would contain:
1) GPS geo-referenced VFR charts (TAC's and Sectionals, forget anything IFR...overkill)
2) Nicely decoded (AKA human readible) METAR's and TAF's at the airports en-route, of course, it would have to grab that updated data any time it found an internet connection either via WIFI or other connection.
3) It would have a quickly accessible "flight flow" where the user can easily or automatically select the next logical function of the flight:
a) preflight checklist
b) engine start checklist
c) before takeoff checklist
d) enroute maps (as discussed before)...this process would also have quick buttons for things like emergency checklist, and frequencies
e) before landing checklist
f) shutdown checklist
g) optional google-maps process for driving directions, etc, etc, etc (to be refined later)
4) While enroute, it would tell you (to the best forcast it could retrieve), the current winds aloft (and if it's a head/tail wind) at your altitude, and the altitudes above and below you...allowing you to make real time informed decisions. It is important this functionality is graphically presented instead of numerically presented.
5) Anything for maintaining passenger interest, such as:
a) points of interest around the airplane that you can point out...my passengers are always asking "what is that over there". This would be great if it could be blue-toothed to other devices in the cockpit so the passengers can see this directly on their own phones. This would mainly include prominant landmarks like cities/towns/lakes/windmill farms, etc. No need for power lines and train tracks.
b) a quick way to enter a GPS coordinate while sitting in your passengers driveway (or as provided by them before hand) and a google maps image of that point on the earth so you can easily see thier house and fly over it in the optimum position for them to see.
c) along the bluetooth streaming data concept, send the current altitude to thier phone display so they don't have to decode the altimiter...this data should be both AGL and MSL. If this could include current GPS coordinates that could then be geo-referenced on google maps on their phone, that would be even better.
6) Show the current AGL at the current location...a lot of times I don't care what MSL I currently am...I want to know how close I am to the ground over constantly changing terrain without having to decode from a map...quick and dirty information
I am sure there are lots more...but this is what I can rattle off the top of my head. I will add revisions as I think of them.