I think there will be 1v1 just based on the fact that they said there WOULDN'T be before the game launched, They've made so many mistakes so far that they should realize doing the opposite of what they said is going to be the right decision.
Actually I'd be very happy if they will make something like Battle Grounds in D3, but it'd be too complicating for them.
2v2, 3v3, 4v4 are cool.
But I think 1v1 would be really unbalanced and could be really painful for some players... I'm looking forward for it!