The system I've been busily developing for a military application allows a mission to be planned on a laptop. This includes a global nav database, statistical met database and performance databases for the aircraft and those with which it will be working. The system also prints an associated ICAO flightplan.
Once the mission plan has been drafted, the planner can adjust it and replan using actual met-of-the-day. Then it's downloaded to a normal USB stick. At the aircraft it's uploaded into the onboard system, which uses the same software as the laptop. It also displays the actual CG within the allowed CG envelope.
In flight there is a fuel graph which shows actual fuel against planned fuel, a moving map and various text displays. All update in real time and the plan can be edited and amended as required. Due to certification restrictions, the system is only allowed to receive aircraft system data from an ARINC bus; it could update the FMS but the customers didn't ask for that functionality - which would have required intensive and expensive testing.
There's no reason why such funcionality can't be extended to the GA world. Plan in the warmth of home, update the plan on the day of flight, feed it to a USB stick, upload it to your GPS.