We quickly need Viewpoints to be improved by a couple of adjustments to existing related functions.
Given that the Viewpoint idea would be to allow us to display only what we want a client to see, the first part of the problem is that the Viewpoint is defined by the 'display', rather than by a 'window'. My model space has a 'landscape' orientated rectangular shape, but if I want to show a square room, or a 'portrait' orientated area, when I define a Viewpoint there's unwanted areas 'captured' by AGi32.
First thing we need is for Viewpoints to allow a 'window' definition.
Once in Page Builder the problem gets worsened. A Viewport can display the Viewpoint, but it does not centralise the image - rather it attaches it to a corner. It also does not allow any 'fit' functions. This further tends to exacerbate the first issue, since it's possible that our Viewport is dimensioned such that there's even more unwanted areas shown because of the mismatched proportions of the Viewport and Viewpoint.
So the next thing we need is the ability to fit the Viewport to the proportions of the Viewpoint, lock the Viewpoint to centralise in the Viewport, and allow proportional scaling of the Viewpoint in the Viewport as we re-size it to fit the Page Builder display.
As mentioned, this needs to be a quick improvement - the recent major release shows that you're pushing the concept of Viewpoints as being an improvement over other related display methods. In fact I've found issues with my old work due to the changes you've made. The Viewpoints concept has a lot of potential merit, but it needs to get tweaked in order to make it viable.