Are you looking for a way to represent stencil elements in a way other than their actual appearance in the diagram? Check out this demo example that shows a different representation of elements during the drag-n-drop sequence (template-preview-diagram).
Drag elements from the stencil onto the canvas and watch how their appearance changes.
The source code of this demo is available as part of the JointJS+ commercial license. Don't have a license yet? Start a trial and use the source code of this and many other demos for free, with no obligations, for 30 days.