Be able to choose a starter from a wide amount of choices, in various types, and, depending of their type(s) (ideally fitting its species precisely), said starter having a personality, many story-driven interactions with you and other NPCs (and dialogues one way or another), as well as a long and deep sidequest to complete, and even moments when we get to control them directly.
How would you have the Pokémon be able to really communicate with humans in a future game ?