Hi,
I would find also comfortable and it would improve user experience to be able to create a UML-RT state machine diagram on a capsule, with the implicit creation of the RT-state machine and all usual content (region/state/initial/initial trans). And at the same time, the ability to create a plain state machine diagram should be removed.
For the creation of a new sub-menu dedicated to UML-RT diagrams, I am not sure that it may be of interest. I would have rather imagine a "new view" menu, with all tables/UML diagram/UML-RT diagram/ Further customized UML-RT diagrams at the same place, and its content filtered based on the active viewpoint(s).
I could imagine that several viewpoints would exist for plain Papyrus-RT, at least 'basic' and 'advanced'. With basic, you would find only the new UML-RT child menu and only access to UML-RT customized diagram, advanced would had the new UML one, and access also to plain UML diagrams. So beginners would have a quite straight user experience, with less available menus, and experts would still have access to the full UML stack.
But I know we have been discussing of that UML-RT specific menus quite a lot in the past ;-)
Cheers,
Rémi