Kevin or Scott can speak to the number of web workers used, and I’m sure provide more details to my comments below.
‘Allow imagery detail to increase independent of increasing terrain detail’ . It would be great to decouple these
This is the long-term vision for sure. In the short-term, the plan is to at least tessellate areas without terrain using the ellipsoid model so the imagery is able to refine. We need this for an app we are in mid-stride on (app details to become public in about a month).
‘Client-side (approximate) level-of-detail control.’ assuming this would allow e.g. control of how finely terrain gets tesselated based on client hardware etc.
This is to control screen-space error, which is a speed/quality trade-off. Although I see the property in CentralBodySurface, it doesn’t look like it’s exposed yet.
Currently, we do not tessellate terrain beyond the true data, e.g., apply fractals; however, it’s not out of the question for the future. Is this what you are asking about?
‘Show terrain in Columbus view.’ In terms of UX it could be like looking at one of these 
We also need this for the app we are working on. In general, we think Columbus view has a lot of potential given its coolness factor, and we have yet to fully exploit it. If I can put words into Kevin’s mouth, he assures me it is not hard (however, it is not trivial either; for example, things like horizon culling need to be revisited, and horizon views in general have the potential to be troublesome). We’re hoping to have it in December.