Mew The Gato
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I would love if they showed us the next stage of evolution for the Kalos starters but kept the final stage secret until we play the games. That way we could have a better idea of where the evolutions are going but also get a nice surprise for when we play the game ourselves. That would be my hope anyway.
As for what I think we'll get in CoroCoro, I would say the Fairy Type chart, Kanto starter Megas again, 3 new megas and maybe 3-5 new Kalos pokemon hopefully including Oorotto and llama officially revealed.
I agree. The middle evolution is usually the least iconic, though, so they are more likely to reveal the Final Forms if anything.
Although your idea would be much better in my opinion.