John,
The answer is simple, everything a user can do manually in layout!
Just a short list to get an impression of what Skalp would like to do with layout api:
Attach models
Update models
Remove models
Define the paper size and the template
Place new model views
Define the size of the view
Setup the scene of the view
Setup the scale of the view
Setup the render mode and lineweight and color
Create layers and control this layers.
Place dimensionlines
Place line, shapes, …
Control color, weight, layers,… Of all this objects
Insert images and control these images.
…
I think it easier to ask what we don’t need… But the problem is there is nothing at this moment I can think off we for sure don’t need. Ok I can give you a list of more important features if you like. But if you like that, I need to think about what’s the minimum we need to do something good with layout. But be sure gives us an API, and sketchup + layout would become a totally new level!