I'm sure they will find ways to include several game types. Who said that the game had to be one way and one way only? For all we know, they will be including game types we have yet to even hear of, so its too far fetched to assume the game will forever work in one single way.
I don't know, but looking at how Blizzard looks at PvP, I don't expect it. It would be quite easy, but they won't bother. Heck, you could also have other interesting stuff like Capture the Flag or other unique gametypes. But who cares about PvP, right?