You really underestimate Blizzard's ability on pulling out content out of there asses, and I mean that in the good way.
For what it's worth, after we deal with all the obvious plot points, who's to say we won't be traversing the vast universe?
Sara Kerrigan as a final boss someday.
Diablo as a final boss someday.
Donald Trump as a final boss someday.
I can go on.