Here's what editing a template part in isolation (via the sidebar menu in the Site Editor) looks like currently:

Here are a couple of opportunities for improvement I see:

• The boundaries of the template part are not clear.
• The template part occupies the entire width of the canvas, parts that render in narrower containers look odd in this context. Now that the canvas is inside an iframe perhaps we can add resize handles?
• The "Template" tab in the sidebar should read "Template part" and contain an option to set the name and area.

Any other thoughts?