This is incomplete but still indicative of the direction I'd like to see Mosaic 3.0 move towards. Putting it out there in case it inspires someone.
I especially like that the right sidebar with available custom tiles displaces the full width of the site, similar to the way the edit bar does on the left in standard Plone.
I've added some additional annotations below to help to explain my ideas:
The mockup file is available here: