Change the listview for an iconview/grid wallpapers (filenames really don't matter for selecting a wallpaper)
“Open folder” as alternative to “open/add file” (many people keep their backgrounds in one folder)
Drop the brightness and saturation meter (xfdesktop != image-editor)
“Hide” additional options for images in an “advanced” popup or tab (at least: auto-change every $n minutes, file-list)
Alternatives for tabs for additional displays? GtkCombobox (pro: consistent with display-dialog, con: not obvious whether there are new/additional displays)
Mac
OS X uses individual dialogs for each monitor.
We can bundle background styles in a single combobox:
Drop the Auto option.
Add “None” to hide the image.
Would that expose/enable additional buttons to add/edit the background color/s?
To avoid a regression we should always show the possibility of a gradient (horiz/vert), so the old transparent svgs works with a gradient.
Add the single screen image options: Tiled, Centered, Stretched, Scaled, Zoomed.
Add option “Stretched”, to span across screens, disabling options for 2nd tab. (+1)
Possibly put a separator in the menu for these 3 groups.
What about per-workspace wallpapers?
A much asked-for feature. I think it'd be nice, although I'm not sure yet how to bake it into the UI without making it messy again. (Similar issue as with the multiple monitors in fact, so maybe there's a common solution to both.)
A nice feature for wallpaper transformations would be a fade-over.