Thanks for your response. Densifying the line isn't really appropriate as it takes a large number of points to make the curve sufficiently smooth, and even then when zooming in the points on the curve as so far apart visually it begins to look less and less curvy.
I've managed to get something working using the ElementLayer but it took some effort. The curve is defined by a QuadraticBezierSegment within a Path, so I need to specify 3 screen points (start point, end point and control point) getting the right interplay between the map points and screen points has been tricky.
In order to fix the curve between two map points, I determine the envelope covering those points and pass it to the UIElement to be drawn in the ElementLayer. The UIElement consists of a curve in a Viewbox so it can be scaled appropriately as the extent changes. However, the results aren't great and I need to change the stroke thickness each time the extent changes.
I'm surprised that simply specifying a control template for a symbol does not work. Is there any intention to allow for complex paths to be used in control templates in future versions and if not, can I suggest this as a feature to be considered?
Drawing curves seems to be such a natural thing to want to do, but it's been made really hard!
