The gameplay (from a very general perspective) hasn't changed much from expansion to expansion. It was always....Level > Dungeons > Quests > Raid > PvP.
That said, I can't think of many other extensions to the game that could be added.
It also wouldn't make sense for Blizzard to "move on" from WoW and focus on Titan. With ~10 mil subs, WoW is still one of the top revenue generators for Blizzard (if not THE top). Regardless of declining subs, it doesn't make financial sense for them to take resources off the game when it's still thriving. Also, considering that they don't have key players working on multiple games, I would assume that Titan has their own dedicated team. WoW wouldn't be able to function smoothly with a skeleton crew, so it would be ill-advised for Blizzard to shift WoW resources to Titan.
I'm sure the heads over at Blizzard don't foresee WoW dying any time soon. Titan (when it's eventually released) will most likely be a new market for Blizzard to reach out to...very much like Starcraft and Diablo. From a financial standpoint, it makes no sense to cannibalize their own fan base.