I use Goodnotes for idea generation for work and the one thing I miss from my previous app for this (Forge) is the ability to move pages around on a kind of pinboard grid, placing them wherever I want (in groups, solo, rows, columns, with space between them if desired). It would allow free-form placement of the pages on a grid perhaps 6 pages deep and as long as it needs to be, and would be an alternative mode to the traditional notebook sequential page style you currently have. Instead of opening up the sequential thumbnails view, it would open up the pinboard view, but you would move the pages around in the same way. You could choose to view and work on the document in either mode, and switch between them as needed.