If they give too many pokemon Megalutions, it will lose it's importance, and eventually all of them will get one. They need to be selective, for sure. That doesn't mean they shouldn't introduce some new ones in ORAS, but simply to be cautious. Obviously, Swampert, Sceptile, Groudon, and Kyogre are all premier candidates to receive mega evolutions. I could see pokemon such as Metagross and Salamence getting new forms as well.