Plane Plane Intersections
Here is a little demonstration of Plane/Plane intersections. This is all calculated mathematically and not using any built-in Revit API functions (other than dot/cross products). In Dynamo this is fairly trivial, but the Revit API doesn't expose Plane intersections (unless you create faces - which seems more like a hack), so thought I'd create my own functions.
Since Plane/Plane intersections return a line of indefinite length I have also bound this line within the extents of a BoundingBox that circumscribes all the Elements in the model, this means that the resultant line can be drawn in context to the model which is handy and isn't too short/long. I created a little box family to visualise this in the model which looks pretty nice!
I'll share the code in a C# helper library at some point and will probably abstract so it can be used Unity also and is not limited to just the Revit API.