There are still mega evolutions to be discovered in Omega Ruby and Alpha Sapphire. The article is in Portuguese so I just did a google translate with some fixed words (makes more sense):

Eurogamer : Are there more mega evolutions to reveal?

Shigeru Ohmori : Yes, there are many Pokémon with mega-evolutions in Omega Ruby and Alpha Sapphire, and some have not yet been revealed. We expect players to explore the game and find out for themselves.

Eurogamer : Is Dragonite one of the mega evolutions to be revealed?

Shigeru Ohmori : You like a lot of Dragonite, do you not?

Eurogamer : Yes (laughs).

Shigeru Ohmori : I want to save some surprises for the game, so you'll have to play to find out.

>I want to save some surprises for the game, so you'll have to play to find out.

> You'll have to play to find out.

Link to the original article in Portuguese: http://www.eurogamer.pt/articles/2014-10-15-entrevista-pokemon-omega-ruby-alpha-sapphire-nova-habilidade-soar-mega-evolucoes-longevidade-e-a-new-3ds