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More of an opinion than a fact, although I did like that gen for what it was. I agree however, I would have preferred more Pokemon, but also more actual evolutions to improve certain Pokemon that could do with an evolution (Like they did in D/P/P). Megas do make things unbalanced, but all they need to do is make the use of Megas restricted e.g. Megas being allowed/disallowed in certain matches based on the options chosen.
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Well, what's the point of creating Pokemons that are going to be banned on tournaments? To make them just for fun? Therefore, don't do them, it is a waste of time, and worse... it is a waste of ideas. A nice and more acceptable way to remember of Pokemons from the previous generation is by creating pre-evolutions or evolutions, like Gen IV did. We have a lot of standalones that really needed an pre-evolution (Laparas, Pinsir, Heracross, Skarmory, Kecleon, Tropius, Zangoose), and some crappy ones that needed an evolution (Dunsparce, Maractus, Weezing). Kanto Pokemons will never be forgotten, and they were the ones who most received mega-evolutions. Bad way to remember them... Just make them availabe in the Kalos dex, and it was already enough.