We don't need OS 11. Just think of OS X 10.0 as a new start and different kind of Mac OS, which it was. OS X 10.9 is basically the OS 9 of this new Mac OS generation. So OS 10.10 will be like 10.0, 10.11 will be like 11.0. There doesn't need to be an "OS XI" or "OS 11". OS X is already robust as it is. The only reason X was any different was because it was such a big departure from the classic Mac OS. The X and "10" is just branding. Especially since the X initially signified that it was Unix based whereas the Classic Mac OS was old and busted.
There's no reason to make it graduate to 11. If there ever is an 11 it will be because the architecture is so radically different from what we have now that it requires a new version number. Until then, 10.11 will be our version 11.0.