There is one big problem with themed naming schemes, and that's running out of good words. While OS X has had a good run, evolving from Cheetah, through Jaguar, Tiger, Leopards and—most recently—Lions, the future looks bleak.
The problem is that Apple has used all the best felines. Even the shift to Mountain Lion for the most recent iteration of OS X is a demotion: the mountain lion is, strictly, part of the puma genus, and could easily be taken in a fight by the current, real lion.
Any further demotion would be an embarrassment. At any rate, Lynx has been used in the past for the hand-held Atari console, and Ocelot is already taken by Ubuntu. And dear God, names with the word "cat" in them are a complete a non-starter.
That only really leaves, from the commonly known felines, the cougar and extinct saber-toothed tiger as possible naming options for future releases. Maybe it's time to change animal family, Apple? [XKCD]