1. Apply button in Settings;
2. Resizing of DF taskbars;
3. If #2, sync windows taskbar size with DF taskbars (with option do disable it).

4. Integration with "2 monitor" and "3 monitor" sections of vladstudio.com (ability to change 2(3) walpapers together if image is big enough).

I have, for example, notepad on my second display.
I press CTRL+WIN+~. It span across two monitors, ok.
I press CTRL+WIN+~ again. It restores on default monitor (1), restores with almost fullscreen size (2).

My suggestion is:
5. to save some window information in memory. Thus if i press CTRL+WIN+~ snapped window restory on owned display with such size as it had before spanning.

I think that is not a problem to identify unique windows even with alike captions. If not, i can loop up some in MSDN+google for it.

10x!