My fantasy UI would be a sort of large flat desktop plane, with ~infinite zoom. FVWM and other do have the one large desktop space that you scroll around, instead of discrete virtual desktops, but I want to see the ultimate version of that.
A vast virtual plane that you could scroll and zoom around, drag your files/content around, make groups with marked delineations like a map. Obviously there would be some kind of grouping and minimap if you zoomed out enough. Documents should be represented such that you would simply zoom into them to access their content, you wouldn't open files or start applications, everything would be represented visually and the editing tools you need would be presented as you zoom close enough. A document would be represented as pages you could flip through or spread out for a larger overview. Videos could be simply viewed or spread out into frames/segments with editing tools.
Content would not be a "text file" or an "image file", it would just be a delineated piece of content, which you could add whatever type of content to, by calling up the relevant tools.
There would be no discrete applications as such, just content and flexibility, I want it to be seamless and integrated. In some ways similar to UIs seen in Minority Report and other sci-fi movies, but without the Hollywood flash and gimmickry.
I'm aware that there are a lot of complications and practical limitations, but it's an idea I've toyed with for a while, and I'm not sure I've seen people really attempt something like it for practical use, only bits and pieces.