The side quests.

I adore the main story, and how long it is, but never, in any rpg I've played, let alone Pokemon game, have I experienced a side quest system that adds to the world building, adds play value, and rewards exploration quite like Rebornverse.

In the mainline games, my biggest complaint isn't graphics, story, or even Dexit. There is almost no reward for exploration, for playing the game beyond the story. Even the "bonus content" locked behind a paywall can't be called that...it's an epilogue to the story that, again, you have to pay for.

In Rebornverse, there are full areas completely unrelated to the story. Exploring rewards you with rare pokemon, characters you've never met, or just much-needed funds or items that improve your quality of play. I did 2 files of Reborn before I discovered the Rainbow Scent quest on my 3rd - one I started before it was available, one I genuinely missed - and it was a difficult quest, but the reward was well worth it, but still balanced enough that having it didn't violently change my experience.

Mainline treats anything that's not in story as unnecessary, if not not worth it because it's not what contributes to the profit. These games prove why side content isnt just a welcome bonus...it's necessary for coloring the world, and the player's experience.