Niantic appears set to introduce 100 new Pokémon to mobile hit Pokémon Go in a forthcoming major update, after references to every second-generation character were discovered in the game's code following its most recent update.
Character models and other data have yet to be added, but references were found in v0.45.0 to every Pokémon that was introduced in Pokémon Gold and Silver in 1999/2000, from Chikorita (#152) to Celebi (#251) (via TheSilphRoad).
There are placeholders for second-gen legendary Pokémon Lugia, Ho-Oh and Celebi as well, despite first-gen legendries Articuno, Zapdos, Moltres, Mewtwo and Mew not yet being made available to capture in the hit augmented-reality mobile game.
Ditto is the only other first-gen Pokémon that hasn't yet been made available (making the current number of available Pokémon 145 rather than 151) but a reference was also found to Transform: a defining move of Ditto's.
Pokémon Go was released in July and quickly became a worldwide phenomenon. Interest has waned since (which is to be expected), but it remains one of the highest earning mobile apps in the world, having already netted developer Niantic and its partners more than half a billion dollars in revenue to date.
With 721 Pokémon in total (a figure that will grow with the launch of Sun and Moon later this month) Go was always going to be updated with later generations to coax players back.
You probably shouldn't expect to see parks fill back up with excited players or further stories of people discovering corpses or breaking into private property, however. Pokemon Go does still has an exciting future ahead of it though, with the promise of new Pokémon and features such as battling and trading with friends.