Soooo many places in Kanto weren't in the games tbh. It's almost hard to pick.
I'm gonna pull up Bulbapedia specifically for this reply, because I can't think of them all.
If anyone else wants the list: https://bulbapedia.bulbagarden.net/wiki/List_of_anime-exclusive_locations
UM, narrowing it down to a few:
I really liked the palace / Rota in the Lucario/Mew movie!
Also, New Island from Mewtwo Strikes back.
All of the Orange Islands were cool!
LaRousse, which is weirdly not in the anime, since that's where Drew is confirmed to live.
Lastly, the Unown Dimension seems like a neat concept.
& I did love the Mirror World in Kalos, I wanna see more of that tbh,