As far as lore goes... sargeras is the big baddy. King of all the baddies. We had an entire expansion revolved around him, and we didnt kill him. Instead we killed the most powerful demon hunter known to azeroth/outland... good job guys!
So now i assume when he shows his face, if we manage to destroy Sargeras, will that be the end of WoW? He's the most powerful force in this universe and the next, and mostly everything we've done up until this point can be traced back to him. The scourge, forsaken, warlocks, demons, Kiljaeden, blood elves, old gods, nearly every evil we faced was directly or indirectly caused by the burning legion.
I picture that a battle with sargeras would destroy azeroth, becuase if he reaches us it's pretty much game over. Plus, how would he even get here? The portal in outland?
But.. after we've "dealt" with sargeras, what else would we do as a player? The old gods theme has been beaten to death this expansion, and the last.