How to create custom geometric object?

It should have the following properties, resembling properties of built-in geometric figures, like Circle etc:

1. Has constant head, say RoundedRectangle. It should not evaluated to the list of lines, so as normal Rectangle.

2. Can serve as a subject for GeometricTransform

How to create custom Graphics primitive?

It should have the following properties, resembling properties of built-in geometric figures, like Circle etc:

1. Has constant head, say RoundedRectangle. It should not evaluate to the list of lines, so as normal Rectangle.

2. Can serve as a subject for GeometricTransformation