Allow separate windows for the same source document
For multi-monitor setups allowing child windows to be separated from the single host app window would be extremely useful, as per Excel: https://blogs.office.com/en-us/2013/02/07/open-excel-workbooks-in-separate-windows-and-view-them-side-by-side/, or, ideally, using Visual Studio's tear off windows, out of the main app into some spare screen space.
Although you can stretch the app window across multiple monitors the break between two monitors never sits in the right place (ie the break between two child windows) and so you end up using a single monitor.
In Visio, it's common to have multiple ShapeSheet windows open at once, either displaying master and instance, or one for each sub-shape in a group. In my setup (28" 3840x2160 @ 125%) I recon on one drawing window + a maximum of five ShapeSheet windows as being usable, but beyond that you spend all your time scrolling and switching windows. One of the real benefits of a large screen in Visio is being able to see more ShapeSheet sections at once and this is limited by only being able to use a single monitor.
From the drawing window perspective, other scenarios might also include: a) master details view - for example a one window for a floor plan, displayed alongside other windows of the same page at different zoom levels and areas of focus, b) multi-page view - ie one window per page
Bottom line - I've got plenty of screen real-estate and I'd like Visio to make full use of it so that I can be more productive.
John Goldsmith commented
Yes, good point. Also the handling of monitors with differing dpi's is now much improved too. https://blogs.windows.com/buildingapps/2017/04/04/high-dpi-scaling-improvements-desktop-applications-windows-10-creators-update/
One further benefit that I forgot to mention is that by separating windows you get use native windows snapping, whcih makes life much easier for large displays.
John M Visio MVP commented
In the old days, multiple monitors were not common and when they were, the monitors were the same size, so extending the screen across them was relatively easy. Now, with smaller laptops and tabloids that are connected to external monitors, there are screen size differences, so extending a screen is not a pleasant experience. Separate monitors gets around the size differences.