Semantically, Cougar might be problematic (Cougar and Puma are the same animal) but it doesn't really matter--for instance, there's no actual cat species called "panther", it's just a generic name for big cats, and the black panther is just a subspecies of jaguar. (However, interestingly enough, snow leopards are a completely different species than leopards.)
There's other synonyms for "Cougar" you can put in there ("Mountain Lion") to extend the queue. In fact, Mountain Lion might be a minor upgrade on Lion just as Snow Leopard was a minor upgrade on Leopard.
EDIT: Fixed taxonomical error.
EDIT: Turns out "black panthers" can be melanistic jaguars OR leopards.
I didn't suggest it as a product name; I just said "Panther" and "Jaguar" both describe the same animal, so it shouldn't be a problem for "Cougar" and "Puma" to describe the same animal.
Incidentally, "Panther" was the first Mac OS X to be released in a black box instead of a white box, and they used shots of black panthers (the cat, not the political group) to promote 10.3. I think they were obviously playing with the allusion even then.
So go with Lion, Mountain Lion, Lynx, Serval. Then it'll be 5-6 years from now and either the slang usage of "cougar" will fall out of use or at least the novelty will be gone and people will interpret it as an allusion to the cat itself.
At this point I can't see Apple keep using the OSn (ie: OS11) convention because "OS X" has become such a powerful brand in itself. I doubt they'll want to give that up.
My guess is that they'll stick with 10.x for some time and might eventually name their next major OS something like Mac OS X 11, even though it sounds silly. Or what about xOS (vs iOS)? Has a nice ring to it :)
I'm sure there has been countless meetings at Apple regarding this. I wish I was inside Steve's head...
Most people I know call it OS "EX". In fact, I'm one of those people, even though I know perfectly well it's OS "Ten".
OS 10 doesn't ring well with me, perhaps because I never really used any of the Mac OS versions that preceeded it and feel that OS X is not from the same family (being mostly NeXT with a reworked UI).
Indeed. Most people don't realize it's "OS Ten". Kind of like X11, OS X is probably the last "major" version of MacOS as we know it. By the time they move away from 10.x they'll probably be ready for a new name. I don't quite see how it'll work but lots of people think iOS and OS X will merge.
Apple's running clear out of cats, so I sincerely doubt that they would start off OS XI with a cat. I've seen constellations suggested as the code names for the next series of OSes, which wouldn't be out of the blue considering OS X's recent use of starfields and such for marketing material and desktops.
It has been known for quite some time now that the upcoming iterations of Apple's operating system will be using HP Sauce's 57 varieties as a naming scheme, allowing for an unprecedented amount of revisions before trying to think of something else.
That thread is linked to an Engadget post, now off the front page but with 107 comments as of this writing, compared to 18 here.