Calling it now, and some of these will be a bit ambitious:
> Wallace is the eighth Gym Leader, Steven is the Champion, and you'll be able to face them in a Double Battle at some point, probably postgame. Both of their aces will receive Mega Evolutions (Milotic and Metagross)
> Juan will show up at Master Rank Contests, and can also be battled somewhere in Sootopolis City daily, probably with his final rematch team from Emerald, and he will reward you with expert Contest advice and maybe even some Berries that are used for strong Poke Blocks
> May / Brendan will be fought more throughout the story, fully evolving their starter by Lilycove, and at least one more battle after (probably postgame) where they will Mega Evolve their starter
> Magma's base from Emerald will return but only in Omega Ruby. Some contrived plot device at Lilycove will make you turn back and head to Mt. Chimney, possibly Steven letting you know that Team Magma is afoot but he isn't sure where to find them so he suggests going there, meanwhile in Alpha Sapphire you storm the regular Aqua base near Lilycove
> Team Magma and Team Aqua will both try to awaken their respective Legendaries in both versions, both are successfully awakened, and Rayquaza tries to intervene, but the cover Legendary for your game is the one that ends up becoming Primal and sends the other and Rayquaza off at bay, while the team leader who tried to summon the Legendary tries to control it, meanwhile you're suggested to go catch Rayquaza or you can go after the team leader. You'll face the Primal Legendary in a manner similar to B/W Kyurem and Ghetsis from B2W2
> Wally will have a Mega Gallade and could possibly team up with you at some point for a double battle, like if the Steven and Wallace battle would be happening, leading to a 12 vs 12 brawl
> The Deoxys, Ho-Oh/Lugia, and Mew islands will be available somewhere, as events
> And the most ambitious: Sevii Islands are available in the postgame
Hoping for at least one.
