Actually, a tiger is probably much more powerful than a lion, and certainly more so than a leopard, so there really isn't any relationship between the specific cat and where the OS is going. Also, Apple isn't locked into cat names for Mac OS releases, so even if there were no more cats, they can just switch the code name to something else.
(Re-)Unification of Mac OS and iOS is not imminent.
The choice of the term Lion is very symbolic. I felt that 10.7 would be the last iteration of OSX before they announced it, the name simply feels like confirmation to me.
There are plenty of reasons to expect Lion to be the last version of OSX:
1) iOS is going to gain more and more features over the next few years, bringing it closer and closer to the capabilities of OSX.
2) Mobile hardware will become much more capable over the next few years, allowing the hardware to run a full desktop OS.
3) The OSX UI is getting more and more features and not all work that well together. It feels unfocused and inconsistent and will continue to feel that way until they start with a clean slate and drop some of the older UI elements. Lion appears to be the testing ground for some iPad friendly elements that may eventually for the primary means of navigation on the mac.
4) Windows 8 is supposed to be Microsoft's first OS really designed around touch. it should be out around the same time as Apple's next release after Lion.
5) Macs and iPhones running the same OS would instantly destroy Microsofts desktop OS near monopoly. Add a desktop version of Android (not Chrome) into the mix and things get really interesting.
Now I don't think the reunification of iOS and Mac OS is a sure thing, but I do expect a major shakeup of Mac OS because of iOS, and that may include unifying the two platforms. As of today, the iPhone and iPad run the same version of iOS. Apple TV also runs iOS, and I'm sure Apple is working on having it catch up with the iPhone and iPad, despite having a different UI. Is it really that hard to believe that macs may also be running a unified OS 3 or 4 years from now?