The trick with any new iteration of games is to make it seem like they're still offering exactly the same gameplay while being innovative enough to feel totally different. A contradiction maybe, but business wise i doubt developpers or designers MUST stop exploiting the gold mine until it simply depletes from lack of interest by users.
The title can evolve beyond its inherent limits with a few new ingenious features only for yet again another one of your "eternity" preference! GC_ or GC_ , they get an amount of players that grows or stagnates towards oblivion.
Strangely, it's the concept that draws more attention than the tagged version numbers. In anything. Quake-III, included.
Leave the sinking ships behind. -- Fogerty, CCR.