seeing what they did with the Warcraft series it would be logical to follow the same successfull strategy.
Finishing the Trilogy > announcing the MMO before release of the 3rd part of the trilogy > big hype generated > MMO grabs people from the big success of the last part of the series = a new MMO colossus is born.
I really doubt they'll make a MMOG before they finish the trilogy but ofc i'm not from Blizzard and cant be sure about anything.
as of the posibilities... even in Diablo 1 it said that humanity would decide the end of the Great Exile. It would perfectly lead to a possible MMO without having to change much of the race we choose (humans to decide the end of the war). Ofc it would be nice to get some more features as we progress to the ranks of each side but ofc thats plain speculation and doesnt really go in this thread.