Pokémon GO: 10 Missing Gameplay Features We Want To See
8. Later Generation Pokemon
The original Pokemon games for the Nintendo (Pokemons Red, Green, Blue and eventually Yellow) had the original 151 Pokemon.
As of July 2016, there are now 721 Pokemon in the every-growing Pokemon universe. Each new generation of games comes with a new set of Pokemon (and new starters to choose from). No one expects a jump from 151 in Pokemon GO all the way up to 721 immediately, but gradually introducing some of the later-gen Pokemon would give more variety to the game.
Right now the app already tries to mimic the video games by making certain Pokemon more prevalent in different regions of the real world; water Pokemon are more common in bodies of water, urban areas spawn more Pidgeys and Rattatas, whilst rural areas have plenty of Bug and Bird-type Pokemon.
Pokemon GO also changes the Pokémon available in the same area, based on the time of day. In the original trilogy of games, if a patch of grass would spawn Pidgeys, that's all it would spawn, no matter when you interacted with it. In Pokémon GO, you might get Pidgeys during the day, and find a Ghastly at night.
This is a concept introduced in the Second Generation of Pokemon games. Seeing the occasional HootHoot at night would be a nice change of pace from the constant Rattata and Pidgeys.