I'll give you a few of the features that ORAS has to offer based off your questions, but I'll try not to spoil too much xD
New Megas: Yep! Some examples are Mega Sableye, Mega Altaria, Mega Salamence, and of course the Primals... There's a good amount
New Features: Well, there's a new BuzzNav feature... Basically a 'news' sort of deal. And the DexNav, which is a new thing that helps you find Pokemon better, even some with special moves unobtainable without breeding... And the three original options (PSS, Super Training, Pokemon Amie) are still there.
New ways to battle: Uh... Other than the Megas, Primals, and the fact that Mega Rayquaza doesn't need a Mega Stone to evolve, not really.
The new moves make a difference too, but the battle style has stayed the same from X/Y.
Shiny Blocked Pokemon: You're right. Some Pokemon can be obtained legitmately in ORAS now. A perfect example of this is Shiny Reshiram and Shiny Zekrom xD
Shiny methods: I think they've stayed the same from X/Y. I may be wrong though
Hoped I helped!
Those are the ones I can think of off the top of my head
Trust me though, the games are awesome xD