Well I don't know about less inspired, but, as with anything, more and more of the same universe tends to feel stale to some (or most). I wouldn't agree that a large, well, cataclysm that somewhat reinvents the map of WoW is retreading the same territory (as you probably assumed while you were writing that).
Haha yea I'd agree with you there. Everyone knows about WoW. It holds about 60% of the MMORPG market. And I would guess a more accurate tracker (as Don said, that site only records people who actually have that addon) would show similar rises and dips for every xpac and content patch. As the game ages and more people become more familiar with its content I'm sure the dips would become more dramatic, as less people would want to play after exploring whats new, but the overall trends would probably be similar.
I'd guess they'll announce Titan sometime soon considering they've become more comfortable talking about it. If not this Blizzcon maybe the next.
As far as the next WoW expansion, I have no idea where it'll take place or anything, but they announced Cata about 10 months after the release of WotLK, so it could be announced some time this summer (aka Blizzcon.)
It doesn't seem like that particular chart shows it, but I'm sure that if you looked at past expansions there would be a similar trend.
As Scyber said, in the 20th anniversary video they said they want their other other franchises to steal WoW subs, as otherwise they would be stolen by some other company.
Think of it this way, if a game (like D3 or Titan) that Blizzard makes takes 1 million WoW subscribers and sells 5 million copies in total, thats still 4 million net fans for Blizzard, and that also means that those 1 million lost WoW subs probably would have let for another game if they hadn't left for D3 or Titan.