As far as catching Pokemon is concerned, fans are already in the mood to finish their Pokedex with a deduction of an additional regional Pokemon that require travel. This became a reason for trainers to generate an uproar for the new Pokemon that they will be catching so that they will not be bored with the present Pokemon and its features that can be found in the game.
The question is how will Niantic be able to accommodate all of the new Pokemon? Pokemons like Pidgey and Rattata are among the most common Pokemon in the game, so will these new generations have lower spawn rates or will it have increased spawn rates? As of this time, this is a question that is still unanswerable but it said that these will have a lower spawn rate so that it can balance the normal Pokemon that will soon come with future generations, according to Otakukart.
It is also believed that the current rare Pokemon might possibly become more common, although very rare Pokemon like Snorlax and Dragonite will be more increasingly rare since newer Pokemon like Tyranitar and Salamence will transform into a far greater Pokemon. Once the future generation like Gen 3 and Gen 4 will start to break in, the original Pokemon will become rare spawns minus Rattata and Pidgey spawns since they are still quite common spawns in handheld games.
Biomes are also believed to be a shifted or rotated in order to create newer biomes that will be used for future types like Dark Pokemon. Day/Night spawns will be significant since they are introduced in Generation 2 with Hoothoot and others. There are also a lot of things that they are capable of doing within the future game, like Pokemon contests and future battling/trading features. It is still unsure as to what the developers will do for Legendary Pokemon.