Well, I think the heart containers in the DS games, much like in the original LoZ, were given simply because of limitations in the systems. Not enough space for all that, you might say. As far as going from four to five heart containers, I don't think there was actually much to that, just a new idea. Remember that in Twilight Princess, although you needed more Heart Pieces to complete a Heart Container, you came across them all the more frequently. And you always had Fanadi (the fortune teller) to subtly direct you to a Heart Piece you hadn't collected yet.
I personally would like it to stay at five. I think this would help incorperate more sidequests and legnthen the game, too. I don't want cheap sidequests, though, but nothing overly hard. As far as how will we earn our Heart Containers, I think we will see the classic Heart Container after every boss formula, and get other Heart Pieces from sidequests, like we always have. However, I think it would be neat to get some kind of reward for finding a certain number of Heart Containers, like a secret area or something. This is like in TLoS: A Hero's Tale, in which you would have to collect Light Gems. After reaching a certain number of Light Gems, a new area would open up. Although I don't see that really happening, I think it would be cool anyway.