Gym Leader rematches with implementation of Juan as 8th Gym Leader and Wallace as Champion post-game. While it was a nice homage to have Wallace use his Champion team in the Delta Episode, at the end of the day, it felt more like a tossed bone considering we didn't get Gym Leader rematches and Juan. This is made even worse when the Gym Leaders are listed in the Trainer's Eye rematch app but cannot be re-fought.
The Battle Frontier. Another Emerald feature that was disregarded in favor of a subpar post-game plot. Fun battle modes and NPCs with lots of potential for upgraded teams with the advent of Mega Evolution were lost.
Character customization. It wasn't in the originals, but neither was the Battle Frontier in GSC--and that didn't stop Game Freak from implementing Sinnoh's Frontier into HGSS. A lot of great opportunities were lost with character customization not being implemented:
- Instead of having all the Contest Halls across the region give you access to every single contest, which is redundant, they could have been replaced with boutiques.
- Also on the topic of contests, instead of forcing players to go with the, in my opinion, worst contest outfits the series has had to date, players could instead craft their own outfit to use specifically for contests however they liked.
- Instead of players all looking like generic NPCs in Secret Bases, there would be more individuality. In addition, if a player wants to use their "irl friends" as their Secret Pals, they could coordinate together to dress for whatever theme they want the Secret Base to have.
- Character customization could open up the possibility to look like the original protagonists. The "They probably didn't include customization to stick with Brendan and May's iconic look" argument doesn't hold much water when you see just how different Brendan and May look now compared to RSE. But with customization, the original RS and E outfits could be made available for players who want to keep the original-original character looks. And for those who don't care, well, customize your heart out!
- An added segment where the player dresses like a Magma/Aqua Grunt in order to infiltrate their headquarters could have been implemented, along with the added bonus of being able to keep that outfit for the sake of customization (with players wanting to dress up as or join villainous teams as a moderately requested feature). This would also act as a fun homage to HGSS, where not only is the player able to dress as a Team Rocket Grunt to take a team photo in the Goldenrod Underground, but also where the player has to dress as a Team Rocket Grunt to gain access to the Goldrenrod Radio Tower after it has been taken by Team Rocket.
Allow Pokémon to follow the player in the overworld. It would have been a nice chance to bring back the feature that was popularized by the previous set of remakes.
Give a backstory or lore behind the mysterious rings across Hoenn. What exactly are they, and why are they in Hoenn of all places? If this really does involve Hoopa, explain that to me. I get that Pokémon is a multimedia franchise, but I'm not gonna go out and watch the movies or tv anime or read one of the many manga just to get some more insight on the game. If it's in the game, resolve it in-game. As it stands, the legendaries just seem shoe-horned in and like they're handed to the player on a silver platter, when legendaries should be more challenging to obtain and have "legends" surrounding them.
Of course there are other things I would have liked. But out of them all, these are the ones I think were the biggest missed opportunities, and especially in regards to the first two points, part of the reason why ORAS were disappointing to me.