I think for the game to hold my attention long term, they'll have to change up the spawning system.
If they built a time of day schedule for when more Pokémon spawned, it would motivate me to keep the game up after I get my charizard
Like it's cool that pidgey, the bugs, rattata and all those other common annoying ones pop up. That's their purpose, to be common. But if they were to make more uncommon and even rarer types pop up throughout the day on a more consistent basis, it would be better.
Like if I know Pikachu or Squirtle would be at the city part around noon, on my day off I would hit the park. Or if I knew Ghastly was around graveyards/old historic landmarks at night, I would pass by to try and catch one, at night.
Having special events where you can catch high level Pokémon would be dope where I could go catch a Charizard or Dragonite on Pokémon Go weekend at a high level. But only for that weekend. Then it would go back to being a bit harder to find.