Who will be the protagonists?
Brendan and May will once again play as the role as protagonists. Brendan will have a red cap, while May will have a blue bandana by default in the artwork. In the game however, you can customize your player's accessories, in the same way as it was with Calem and Serena in X and Y. Brendan/May will obtain a Mega Glove at the Devon Corporation at some point in the story, which is used to mega evolve Pokémon. The Mega Glove is very much like Korrina's Mega Glove. Also, Brendon/May who battles you four times during the main story will battle you in Rustburo City after defeating Champion Steven, for the fifth time, he/she'll obtain a Mega Glove from the Devon Corporation and will use her Mega Glove in battle to mega evolve her Sceptile/Blaziken/Swampert.
Conclusion: It's been confirmed that Brendan and May are the protagonists and wear a Mega Bangle, not a Mega Glove. And what I said about May having a blue bandana in the artwork was incorrect. But what's been confirmed is that they have a very similar design, but it has been changed and improved though from their original design.
What role will Wally play?
Wally will play the same role as he did in Ruby and Sapphire, which is battling you in Petalburg City with a Ralts, then in Victory Road with almost the same Pokémon as he had in Ruby and Sapphire. When you battle him in Victory Road, he will use his Mega Ring (which is basically the same bracelet-like object that Calem and Serena wear) to mega evolve Gardevoir into Mega Gardevoir.
What roles do you think Juan, Wallace, and Steven will fill?
Juan takes care of a new building that represents a new feature in the game. As for Wallace, he is the 8th Gym Leader in Sootopolis City Gym holding the Rain Badge and will give out TM55 Scald when defeated (since there's no Water Pulse as a TM anymore). As for Steven, he fills the role of being the Pokémon League Champion, as he did in Pokémon Ruby and Sapphire.
Will there be any other League-related changes?
There will indeed be graphical improvements to the Pokémon League at Ever Grande City, each Elite Four's room will have an environment that matches the types of Pokémon they use in battle. Steven's room will look very grand looking. I do feel as there will be some minor level changes with the Elite Four and Steven's Pokémon by about either up by 2 or down by 2. Also the biggest change is that Steven also now holds a Mega Ring on his middle finger (just like Lysandre did), which enables him to mega evolve his Aggron into Mega Aggron when battling him in the Pokémon League for the first time. And when the player challenges the Pokémon League for the second time after finishing the game, the Elite Four and Steven's Pokémon will have Pokémon from all regions (including Kalos), as well as much higher levels.
Conclusion: It has been confirmed in the artwork that Steven has a Mega Bracelet, not a Mega Ring. So what I said about him having a Mega Ring on his middle finger was incorrect
Will Scott make a return? If so, what as?
Yep, he will return and he will serve the same role as he did in Pokémon Emerald, since the Battle Frontier will make a return, despite the games following a Ruby/Sapphire storyline instead of an Emerald one.
What will Rayquaza's role in the story be?
Just the same role as he did in the original Pokémon Ruby and Sapphire, which is encountering Rayquaza on the Sky Pillar rooftop where the player gets to battle Rayquaza at Level 70 and attempt to catch it. The player will also obtain the Delta Orb (Or Jade Orb) upon capturing Rayquaza. But if there are new mega evolutions, the Jade Orb will be called 'Rayquazite' instead.
Assume Kyogre and Groudon receive Mega Evolutions. What do you think
— their types / abilities / signature moves will be?
Mega Groudon will be Ground/Fire, while Mega Kyogre will be Water/Ice. Mega Groudon will have Flame Body ability, while Mega Kyogre will have Refrigerate. And Mega Groudon's signature move is a move that we have not yet known about, which is of course a Fire-type move, while Mega Kyogre's one would be an Ice-type move that we haven't seen yet. That is... if new Mega Evolutions became available... if not, then they'll only gain new formes and would change forme if Groudon held the Omega Orb or Kyogre holding the Alpha Orb, and Groudon would only change type to Ground/Fire and Kyogre change type to Water/Ice, with no changes to their abilities or signature moves.
— their roles in the games will be?
Exactly the same as Ruby/Sapphire. With Groudon emerging in Ruby and Kyogre emerging in Sapphire and appearing in Cave of Origin. Groudon can only be found in Omega Ruby, while Kyogre can only be found in Alpha Sapphire.
— of their Megastones being the Red/Blue Orbs? Or will they be something else?
If new mega evolutions become available, then yes... they would be renamed to 'Groudonite' and 'Kyogrite'. But if no new mega evolutions are made, then the Red Orb and the Blue Orb will be renamed to Omega Orb and Alpha Orb, same would be true for the Jade Orb which would become the Delta Orb. Or they could simply be called Omega Stone, Alpha Stone and Delta Stone, as it looks like from the trailer.
Conclusion: New Mega Evolutions have been confirmed! And Groudon and Kyogre's new formes are not Mega Evolutions, but are similar though. They are Primal Formes. It's has been confirmed that Primal Groudon is Fire/Ground. But I was incorrect about Primal Kyogre being a Water/Ice type though, since it's confirmed that Primal Kyogre is pure Water-type. So far, the items used to make them change forme are not confirmed yet.
What will the Version Exclusive Pokemon be?
Groudon will only be obtainable in Pokémon Omega Ruby, while Kyogre can only be obtained in Pokémon Alpha Sapphire. In the Underwater Cave, Maxie will awaken Groudon in Omega Ruby, while Archie will awaken Kyogre in Alpha Sapphire. Groudon or Kyogre will then appear in Cave of Origin in Sootopolis City, which is where you battle it in it's Primal forme after using the Omega Stone/Alpha Stone to bring it closer and then get to capture it in it's Primal forme. As for in the post-game, Dialga and the Timeline World will appear in Omega Ruby around in the mountainous parts, while Palkia and the Dimensional World will appear in Alpha Sapphire in the ocean parts while underwater. Upon catching Dialga, Palkia or Giratina, the player will receive their orbs respectively (Adamant Orb, Lustrous Orb or Griseous Orb. Which is actually a reference to Pokémon Emerald Version where you encountered Groudon in Terra Cave, and Kyogre in Marine Cave. Also, Latias will only be found in the wild in Omega Ruby, while Latios will only be found in the wild in Alpha Sapphire, they will flee 10 times in wild encounters before battling it on Southern Island.
Will Generation IV - VI Pokemon be available in the wild?
Mainly Pokémon from Generations IV - V will become available in the wild after defeating Champion Steven at the Pokémon League for the first time.
— If so, what Pokemon will be included/excluded?
Most Pokémon that could not be obtained in Pokémon X and Y will appear in Hoenn after defeating Champion Steven at the Pokémon League for the first time.
What new Mega Evolutions will be introduced?
If any new Mega Evolutions are introduced, there would definitely be Mega Swampert, Mega Sceptile, Mega Groudon, Mega Kyogre and Mega Rayquaza. However, I'm actually doubting that new mega evolutions would come in, 'cause there's already Mega Latias and Mega Latios in which cannot be obtained in X and Y unless through powersaves. Of course, Mega Latias and Mega Latios will be obtainable in Omega Ruby and Alpha Sapphire. I'm quite positive that there will be a Primal Rayquaza forme though, activated though the use of the Delta Stone. I also do assume there will be a Mega Aurorus, Mega Noivern, Mega Golurk, Mega Stoutland, Mega Volcarona, Mega Luxray, Mega Meganium, Mega Typhlosion, Mega Feraligator, Mega Torterra, Mega Infernape, Mega Empoleon and finally... Mega Chesnaught!
Conclusion: Mega Swampert, Mega Sceptile were correct. Mega Groudon, Mega Kyogre were incorrect, as they're Ancient Devolutions.
Where will the stones for the new Mega Evolutions be hidden?
If there is any new mega evolutions, they would be hidden in areas around Hoenn and you'd need to upgrade your Mega Glove at the Devon Corporation in order to find out their locations, just as you did in Pokémon X and Y at Anistar City. As for the Omega Stone and Alpha Stone, they're obtained from Maxie or Archie, depending if you're playing Omega Ruby or Alpha Sapphire, before heading to Sootopolis City. As for the Delta Stone is obtained from the Sky Pillar behind Rayquaza.
Conclusion: The player wears a Mega Bangle, not a Mega Glove.
New areas to be introduced?
I'd assume there will indeed be some new areas in which are inaccessible until you've defeated Champion Steven at the Pokémon League, such as the Frozen Island where it actually snows there.
Ideas for post-game content?
Everything of post-game stuff from Ruby/Sapphire as well as Emerald will return in Omega Ruby and Alpha Sapphire. However there are some changes. Latias (obtainable in Omega Ruby only) and Latios (obtainable in Alpha Sapphire only), will be encountered in the wild in the exact same way as it was in Pokémon X and Y with Articuno, Zapdos and Moltres. Except this time, instead of it being based on your starter Pokémon, it's based on which version of the game you're playing. As soon as you encounter Latias/Latios in the wild, it will flee as soon as you encounter it. You need to track it with the Pokédex and encounter it 10 times and then you have to speak to Briney and he will speak about Southern Island and will take the S.S. Tidal over there. Once you reach Southern Island, press A in front of Latias/Latios and you'll battle it for real this time. Anyways, other post game content that you'll get is the Battle Frontier, which is exactly the same as it was in Pokémon Emerald, but with some new minor stuff added in there too, such as Mega Stones which can be obtainable and can be seen at night once you've upgraded your Mega Glove. Also, Dialga, Palkia and Giratina can be caught in Hoenn in their respective worlds. Dialga will be in the Timeline World, Palkia will be in the Dimensional World and Giratina will be in the Distortion World respectively. Dialga and the Timeline World can be accessed in Pokémon Omega Ruby, Palkia and the Dimensional World can be accessed in Pokémon Alpha Sapphire and Giratina and the Distortion World can be accessed in both games when the player has the Hoenn caught Dialga and Palkia on their team. In order to access any of those worlds, the player has to speak to one of the people at the Mossdeep Space Center and he'll tell you about some strange portal appearing along a certain route, while Giratina's one will appear in one area only. Now, as for Regirock, Regice and Registeel, they'll appear in the exact same locations as they were in Ruby, Sapphire and Emerald. However, should the player capture all three Regis, the player will be able to awaken Regigigas inside a snowy mountain called Frozen Island, which is a new area to the game and is an island accessible from the S.S. Tidal. The player can enter Sky Pillar only after finishing the game, however the player will only be able to battle Rayquaza in it's normal forme, after defeating it, the player can be able to move to the back area of the rooftop and obtain the Delta Stone from off the small pillar in where it rests behind Rayquaza.