It would be great if the secondary screen support could be tweaked with an option to choose whether it shows the selected additional view in an additional floating window, as now, or whether it shows it in a subdivision of the main space in the primary window (between the sidebars). The divide could be placed in the centre by default, but be draggable to tweak the balance between, say, a Loupe view and a Grid view.
An additional enhancement to this would be to allow the additional/secondary view area to be positioned to the far left or far right, rather than in between the sidebars.
Rationale: more people are getting ultrawide monitors like a single 21:9 instead of two 16:10s. I just bough one, and while it's great to have so much room for lots of Grid view items, in other views it's mostly wasted - unles you're working on a panorama! It's also usually more grid than you need, to be honest. And Lightroom already has its feature for making use of additional screen space, with some great display options available, but it's inflexible in expecting you to have a second physical display; the only way to use it on a single display is to run Lightroom windowed, snapped to one side, then manually size & position the secondary display window in the remaining space. This is clumsy and also wastes space on OS UI chrome because you can't use Full Screen mode.