Theory: In Omega Ruby and Alpha Sapphire, Zinnia will temporarily harness Rayquaza and orchestrate its mega evolution to end the chaos erupting due to Groudon/Kyogre. Then, you will be able to battle her a first time shortly after beating Champion Steven (similarly to the battle with AZ after beating the Champion in X and Y). In the post game, you will be able to battle Zinnia again to earn the Rayquazite.
Another theory: Rayquaza has both a Mega Evolution and a Primal Reversion, but we won't see the Primal Reversion until Delta Emerald. Primal Rayquaza will have a design very similar to its usual form except a bit flashier (just like Primal Groudon and Kyogre) as opposed to its Mega Evolution which is a rather significant design change.
Another theory: Mega Milotic won't be introduced in Omega Ruby or Alpha Sapphire, but will be introduced in Delta Emerald. Mega Milotic will be the flagship Pokemon of Champion Wallace's team in Delta Emerald.
Another theory: there will be a Delta Emerald.
Another theory: Rayquaza has both a Mega Evolution and a Primal Reversion, but we won't see the Primal Reversion until Delta Emerald. Primal Rayquaza will have a design very similar to its usual form except a bit flashier (just like Primal Groudon and Kyogre) as opposed to its Mega Evolution which is a rather significant design change.
Another theory: Mega Milotic won't be introduced in Omega Ruby or Alpha Sapphire, but will be introduced in Delta Emerald. Mega Milotic will be the flagship Pokemon of Champion Wallace's team in Delta Emerald.
Another theory: there will be a Delta Emerald.