Walter & Mike, thanks for chiming in.
I like either or both of your ideas. Especially Walter's. But I was just looking for something simple, so as to get it done. As they say: perfection is the enemy of completion. Call it a "fat course line" or "enhanced course line" or "course line with theoretical perpendicular glide distance". I figure that once it's released it can be improved upon, and probably would be.
Still, given the choice, what I'd most like to see is a theoretical glide ring around the aircraft while in motion (or while in simulation mode, which would eliminate the need for this fat course line -- although it would be more cumbersome to use it that way for planning because of the back and forth between simulating the flight and then readjusting the course line around). I would really love to see that ring while I'm flying. Because even if very careful course planning was done, the up-to-the-minute glide distance portrayed around my aircraft might mean some course or altitude deviations are in order. This ring could be a simple circle to start with. (Or a squashed circle that points out that a 180 degree turn takes up some of that gliding distance.)
For either feature I don't think there's a need to aim for perfection right out of the gate. I would think it would be obvious that a fat line or a circle are guides and not perfect representations.