More than 2 years ago I made an article to show a way to rotate element using Revit API. Using external events in a modeless form as described in my previous article you can for example make a GUI to get axis and angle from user inputs. It is also using ISelectionFilter as described in this previous article. The new thing is that I use a standard class to store rotation parameters. This way parameters are dynamically feed to methods which are run in external events.