It was surely inevitable but now we've got the first indication that Niantic is planning to add 100 new Pokémon from the series' second generation to Pokémon Go.
The Silph Road has been digging around in the code for the latest release of the game, v0.45, and has discovered the first references for Pokémon from #151 Chikorita to #251 Celebi. These are the Pokémon that first appeared in Pokémon Gold and Silver, which were first released in 1999 and continued the Pokémon story into the Johto region.
The developers also appear to have finally added moveset data for the move Transform, which hardcore Poképlayers will recognise as the signature move of Ditto, the first generation Pokémon that can transform into all of the others - which until now has been unavailable to capture.
According to Silph, we shouldn't expect the Gold and Silver Pokémon to appear immediately - they still lack moveset data, which will require another future app update. But the fact that they are now appearing suggests they will be added to the game rather soon. (Perhaps it will tie-in with Christmas?)
It'll be interesting to see how the new Pokémon go down. Though Gold and Silver (and Crystal, a later addition to the same generation) were wildly successful, they were not part of the same phenomenon of the first games. Given that Pokémon Go has been propelled to success on a wave of nostalgia - could more casual players be put off by the unfamiliar monsters?