imo 3 or maybe 4(dont think so).
there is probably a lot of villains to fight with but not so many new things to implement. We got already too many classes and a lot of races. I doubt that Blizzard is able to add 2 or 3 new professions. As well as it would be difficult to add new exciting content. Underwater expansion would be something new, maybe BL invasion exp also + some underworld. But other continents/lands would be just copy paste from those we all know from Kalimdor, Eastern Kingdoms, Northrend, Outland or Pandaria just another well known tropical/snowy/desert/green/rocky zones to explore with some random races.
and improving graphics and engine in every expansion will finally be not enough for 15 y/o(after more 3 exps ) game.