Mac OS X 10.7 -- the OS that no one is talking about -- should be the next major release of the venerable Mac OS X (since 1989!) So, what will it look like? Seems like nobody knows. All of the focus has been iPhone and iPad for so long that it seems like everyone has forgotten about the old desktop/laptop computers.
Frankly, although Mac OS X is easier to use than Windows or Linux, it's still not what I would call "easy to use". I see not only my parents but even programmers bumble around with trying the locate the right window, the invisible application with no windows open, and lots of UI fragmentation (for example, should you have a "start screen", or open a blank document, or open the last document, or what).
As far as things went in the past, Apple was stuck with that system. For example, if they had moved the menu bar from the top of the screen to the top of the window (like every other operating system ever) there would have been howls of protest from the Mac clan back when they introduced OS X. Believe it or not, OS X was actually a step backwards from OpenStep in many ways. Steve Jobs and the NextStep clan were forced to adopt many old Mac conventions even when they didn't work particularly well.
Remember that the Mac interface was designed for a strictly one-app-at-a time system. That's right, the first Macs did not have multi-tasking, not even fake "co-operative" multi-tasking. So the whole idea of having menu and windows separated wasn't so confusing at first. But then System 6 came along with the MultiFinder and things started to go a little wacky (and note, that was after Jobs left the company).
The iPhone was a blank slate, and so Jobs and the UX gurus at Apple could go back to square one and design an OS that was well and truly proper. Don't doubt that they spent many years on it prior to the public even hearing about the iPhone touch UI, probably since circa 2003, maybe sooner. There were always tablet dreams circulating in the company. I had conversations about it there in 2001.
The blank slate meant they could get rid of all the broken things in Mac OS X. And indeed in all window-based operating systems. Like, floating windows. The original "windows" designs at Xerox PARC didn't float, they were just arranged in a grid. Much simpler to understand. And that indeed is the paradigm used on the iPad, where they are called panels instead.
So... what comes next? I predict that the next step for Mac OS is going to be a major revamp of the UX for desktops and laptops to bring back the best ideas from the Touch UI. I would personally be glad to see the last of the Apple Menu, the File menu, all of the submenus. Most of the Finder I could scrap as well (keep the column view of course :-). No application should ever NOT have a window visible -- that's just crazy. It would be nice if they could sort things out so that I don't have to care which applications are running vs. not running. Maybe they could even -- somehow -- eliminate floating windows. Maybe that's too much to ask.
Since they've been able to break with the past in the iPhone/iPad, I hope that they'll be able to find a way to bring the best parts of the new and integrate them with the old Mac OS X ... the user interface that hasn't changed in any major way in 20 years.
(PS: and I wouldn't count on it being called 10.7 either...)