now if we see Reshiram and Zekrom in the Ying Yang, they are completely opposing forces, the ying yang doesn't represent completely opposing forces, it represents that in everything bad(ying) is something good(the black dot inside the ying) and that in everything good(yang) there is something bad(the white dot inside the ying).
Now seen from the point of view were Reshiram and Zekrom represents the ying yang is still incomplete. This makes me think that the 3rd legendary will give the balance to it, giving it a similar role as Rayquaza.