In older versions of X-Plane the old GPS device was used to set up (and save) a flight plan. Since X-Plane 11 introduced the FMC, you cannot create a flight plane with a GPS device (it seems to me), but only with the FMC. Despite of that I think, you should be able to create a flight plan for a plane that just has ADF and VOR displays (no GPS nad no FMC).
The idea is roughly like this: Use the map (bird's view) to plan your flight, starting at the current airport, then click the destination APT to select your destination (flight plan now is from current APT to selected APT). Drag the straight line towards some navaid to add it to the flight plan. In a profile (side) view, you could add the altitudes for your flight plan. Also allow some editing for each waypoint. Then save it.
So you'd have a flight plan you could use to get an IFR clearance, to load in GPS, to load in FMC, etc.
Another useful feature would be to update a flight plan for the current weather (i.e.: pick different runways).
Many older-than-10-years flight sims have such features; why is X-Plane so very much out of date here?