The X Window System (X11 for short) looks and feels the same on my Intel Ubuntu Linux laptop and on my HP 9000 UNIX workstation running HP-UX 11 on PA-RISC.
X11 is ancient (1987), but it's brilliant. Its designers even anticipated having 16 mouse buttons, and (more importantly) opening a window on a remote system.
The system is flexible, programmable, customizable; I've got all development manual volumes here on my shelf.
While I'm open and sympathetic to any new ideas and improved software systems, including novel windowing concepts, personally, I don't have any unmet window manager needs at present. Any new system shouldn't just replicate a subset of X11, they should go beyond the state of the art to justify the time investment.