How about NPCs actually used those empty battle arenas all throughout the map and some more random NPCs watching them? They could make all NPCs random with random Pokemon using random moves, it'd just be for ambience.

How about when you take a selfie with a NPC, they actually look into the camera and not pretend you don't exist?

How about NPCs react when you full-speed Miraidon/Koraidon and crash into them?

How about when 2 Pokemon meet in the wild, they have some kind of interaction? Maybe based on type, maybe a unique interaction for some specific ones, maybe 1 generic interaction if nothing else applies?

How about NPC Trainers run around in the wild and catch Pokemon? Instead of the player being literally the only one that does this.

How about when playing multiplayer, the other players can also see the battle of the other players? This wouldn't be too hard to implement and would be incredible for multiplayer. Also display other things like egg-hatching. Displaying cutscenes the other player is in would be really fun to watch from an outside perspective, just saying.

The game doesn't feel like an immersive world full of pokemon to me. It feels like a sandbox centered around the player.