OS X 10.9 Mavericks hasn't even shipped yet, but of course there'll be an OS X 10.10 after it, and of course Apple's already working on it. Mark Gurman, however, has been able to put a name to it already. 9to5Mac:
OS X 10.10 is internally codenamed Syrah (yes, another wine), and sources hint that Apple has been toying with a new design across the system that is akin, but not as dramatically different, to the new designs found in iOS 7 and iCloud.com.
I hadn't heard the codename before, but I have heard about the design. Apple spent a lot of time and effort making OS X familiar to iOS users, and all of that work disappeared with iOS 7. It only makes sense that elements of the new design language would also go "Back to the Mac" when Apple can devote the necessary resources to it. How much and in what way iOS 7 gets reflected in OS X 10.10 will likely evolve well into next year, and will be incredibly interesting to see come WWDC 2014.
This year we'll be getting a lot of under the hood changes to OS X - see our complete Mavericks preview - so next year also makes perfect sense for the new interface. Apple won't set an official name until next spring, but anyone want in on the pool early?