Ok, cutting through all the blah blah about a small "revelation" I had today and getting straight to the point:
We'd need to define a horizontal plane in a way that suggests both dimensions in a 2 dimensional plane. We also need to define a vertical _axis_.
We're already familiar with such a model
This very planet! Think:
Two opposite poles define the vertical axis, equator defines a plane _perpendicular_ to that axis!
Let Equator be a circle in isometric view. Draw an axis through the centre of the Equatorial circle.
The point where the axis and plane meet, will be the fixed point for our line.
Now for the actual visuals:
An isometric Equator and points indicating the poles. Then have one curved line from pole to pole. This line can then be moved to any position on the Equatorial line. On that n-s line, a compass rose that connects to the centre of the globe by a line (mast) that shows the _bearing_ (nose is always at 0 deg, tail always at 180 deg regardless of where north or south is). This compass rose can then be moves on that curved line. Where that line hits the isometric Equatorial line, that's the longitude. Key is, the n-s line only crosses Equator at ONE point. No ambiguity about the longitude.
When the user cross-references the intersection on the n-s line and equator, with the line going from "centre of earth" to the compass rose, a 3 dimensional understanding should be possible, I believe.
For the rotation around the mast, that would be the rotation of the compass rose. Since it is a bearing compass, the n-s line would then give the pilot direct visual clues where to push the sticks to move to either of the poles.
In short:
An isometric Equator
Two points marking the poles
A curved line going halfway around the globe starting/ending at the poles that can be rotated west/eastwards
A bearing compass that slides on the n-s line
A line from the centre of the globe to the compass rose
Finally, one crucial bit about moving the compass rose with the sticks.
When the right stick is pushed forward, the compass rose will move in the bearing= 0 deg direction, that is, push the stick forward, and the rose will move where 0 deg is pointing. Stick backwards and the rose will move where the 180 deg marker points. Left stick moves it towards the 270 deg marker and right stick towards the 90 deg marker.
Left stick left/right rotates the rose CCW/CW.