I have heard this before, and it warrants some speculation about what could've been, I guess. Some thoughts I have about the list.
- Going by the National Dex numbers, the ones that could have been in sooner are the Johto Pokemon from Scizor onward (except for Celebi).
- Scizor was going to exist from the start. So Scyther could've evolved in first gen. But he was exclusive to the Red Version, while his Green Version rival Pinsir never could evolve.
- Heracross is here. Perhaps Heracross was supposed to be Pinsir's rival originally, while Scyther and Scizor existed as their own entity without rivals.
- Swinub and Piloswine are there. They didn't get really good until Mamoswine came along, but having these guys around would've been a better temptation toward Seafoam Island then Seel.
- Is it possible that the Steel and Dark types were originally planned and just excluded for some reason? I see Scizor and Skarmory in here, as well as Houndour, Houndoom, Sneasel and Tyranitar. Would've been lame if Scizor was simply a Bug-Flying Pokemon. DAMMIT, GAME FREAK! Why did you take out the types that made Psychic Pokemon not broken?
- Phanpy and Donphan! Awesome! Donphan and Ho-Oh were the only GS Pokemon revealed before Gold and Silver were announced that are on this list, aren't they? Togepi, Marill, and Snubbull must not have been designed at this point yet or something.
- All of the Gold and Silver legendaries except Celebi are on this list. If they were all introduced here, what legends would've enticed us to Gold and Silver?
- Gloom's alternate evolution Bellossom is missing. Maybe that was so the Oddish line could be the rivals of the Bellsprout line.
- Eevee couldn't evolve into Espeon or Umbreon apparently. Were they not designed yet? Or were they just excluded totally for lack evolution possibilities? If they were in this game, I'm gonna guess that the Moon Stone would have evolved Eevee into Umbreon. But there was no Sun Stone in Red and Green. And the Leaf Stone wouldn't make much sense to evolve into Espeon (unless Leafeon was designed all the way back then).
- Dragonite wouldn't be alone as a fully-evolved Dragon because Kingdra's there. But the Gengar line would've still been the only Ghost Pokemon at the time.
- The inclusion of possible 2nd-Gen Pokemon tells me that Gold and Silver are intended to be either sequels or expansions of Red and Green.
- More random thought: Would players have gotten mad if, in the Japanese Blue version (or Yellow version) they actually did fully code some of these Pokemon? I think it would be interesting if in 6th-Gen, the programmers design a lot of new Pokemon but not release several of them in the first two versions, but wait until the third game to actually let players use them. So, the Topaz and Amethyst Versions (or whatever they're gonna be called) might have enemy trainers using certain Pokemon, but the players wouldn't be able to find them in the wild until the "Jade Version" came out. I think it would be a neat experiment to see the reaction.