We already have the 1 or 2 distance rings in the app (not sure how they determine the distance shown). What woud be nice is a "best glide distance" ring. Yes, there's a number of variables and the usual disclaimers would apply, but, based on the glide angle entered against a particular aircraft coupled with the known alitude - terrain height would allow a maximum glide distance ring (label it MGD or similar as opposed to straight distance rings).
No need to show the distance, just how far, thereoretically, I could glide in an engine out scenario. What it's going to give you a very quick visual on is if there are any potentially glideable fields nearby without havign to do those calcs in our heads. In this sort of emergency efficiency is all important. A quick look to see, "Yes, there's 2 fields I can glide to" would be great.
Given that all this data is already in the app, it should just be a case of tying it all together and displaying it. Maybe allow it to be selected off or on in the distance rings menu, independant of other distance rings.