How about a Windows version that includes all of the useful stuff promised with the release of previous Windows versions? I'm guessing at least one person (Bill Gates) would still love to see WinFS, the database-driven filesystem for Windows that was supposed to be part of Windows Vista.
I think the big driver for many people is a Windows version that doesn't have so many ways to do the same thing and with the latest way being the least flexible and most inscrutable (i.e. printer setup and preferences). Perhaps at least one version (call it "ad free") that didn't try to force Microsoft apps on its users.
A thoughtful replacement for DOSSHELL.EXE (renamed EXPLORER.EXE) would be nice too.
I'd like to see Microsoft come out with a standard database application that was part of the OS so that people didn't need to use spreadsheets to make lists and catalogs.