We don't know how things will be handled. Seems like Specialization and Revenants will be Expansion-only content, but that doesn't splinter the player base. The new WvW map is going to be free, I imagine, though I'm not sure if the Stronghold gametype will be. Guild Halls will probably be free, but actually building them might be tied to the expansion.
So, if the new regions and mastery system aren't expansion-only, then there's nothing to split the community at all.
It has the potential to though. It all depends on how they implement the specializations and the subsequent balancing. I can already see a lot of bitching by players like "what? I have to buy the expansion to unlock greatswords on my Necro? dat p2w!!!"
Personally I don't care for the mastery stuff, but I am interested in the new class, specializations, and new legendaries.
Also, I would imagine certain aspects of the mastery system pretty much HAVE to be expansion only, as they pertain to the new map area(again, I doubt they are going to allow non-expansion holders to access that area).