A few things I'd like to see:
1) Auto-stacking. I spent over an hour yesterday organizing a mess of pano and HDR sequences into stacks in the library. I would think this could be automated, perhaps based on image capture time - with user set options to control the process.
Auto stacking on import could drive people nuts as various images would randomly end up vanishing it would appear. Using the spray can to apply a stacking criteria could be useful though.
2) The ability to view, on separate monitors, two different folders or collections in grid view and to drag photos from one to the other (copy or move).
A dual pane file manager would make file managing so much easier. As would being able to put the folders panel on second monitor to use in any module. Currently the second monitor is like a crippled library module if used in grid view.
Though, really one should import files to a well organised location, where they stay forever and for organising images should be added to collections, preferably smart ones. And if you need to place [not move] a file elsewhere, that's what the export dialogue is for. But even so having folders and collection on separate monitors [and thus available in other modules] would be great.
3) To have F11 actually work to show/hide the 2nd window as the menu says it should, but never has.
Shift+E, Shift+N, Shift+G, Shift+ C hides/shows the second window in whatever display mode you want. And cycling through main monitor views [using 'f] does the equivalent of F11 (which is used to toggle full screen on Windows).
4) The ability, with web galleries, to upload minor changes without having to generate and upload the whole thing.
Doesn't that depend on the type of gallery and more importantly how you upload it.
5) Better web gallery templates with more control over text, fonts, image size, etc.
Other people do a range of web galleries for LR. My non-Adobe galleries were created and customised entirely within LR.