No one posted this here yet, so I'm gonna do this.
Some of you may have already heard about this one. In 1997, during a Japanese event (Spaceworld), some lucky players were able to play a first version of Gold and Silver.
You probably heard about the two other starters Honoguma and Kurusu.
Well, earlier today, someone lealed the whole beta, and hackers are working hard to data mien the game.
You can find all the information here
And here is a colorized version of all the new Pokémon present in this demo (all 100 of them).
And I'll also link you to the Johto Map which was radically different
This is pretty big. You can see that there weresupposed to be far more baby Pokémon and evolutions than we had known. Some made their way into gen 4 (notably Leafeon, the last Pokémon in this image. It evolved with a Leaf Stone)
Pancham isn't the first Panda Pokémon, and Slowpoke's shell (referred as Shelder in the Pokédex) was a real Pokémon.
Togepi and Ho-oh also existed on their final form already.
Some Pokémon had radically different design (Politoed and the legendary beats come to mind. Beta Suicune may remind you of something else ("Ed.. ward") )
Honestly, to me this is a far bigger new than Pokémon Let's Go announcement.