A canvas.

The canvas tells me what sizes I'm building from. Since this is an implicit proposition, the Canvas would make this explicit. The Canvas would tell me the 3 sizes of devices I'm building for: phone (@phonedimensionrange), tablet (@tabletdimensionrange), and (@desktopdimensionrange). I'd guess in a few years (@tvdimensionrange) will be a standard consideration.

Tangent: In large displays, I could see the interface being composed of many smaller interactive components, as well as a large scale or high-resolution images and interactive visualizations.

With a Canvas, I can drag from a Toolbox of Components.

Each Component can interact with another component, several components, or a backend server (via HTTP REST).

Given that a blank canvas is an implicit proposition, provide working boilerplate (templates) for a few basic styles. See Boostrap Templates

Making the implicit explicit is a characteristic of intuitive behavior.