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The way I see it, every new Pokemon must have:
>A dex entry
>A new design
>A unique movepool
So to me, Megas don't count. Do they legitimately add something to Battling, Trading, and Collecting? Definitely, if you can trade Pokemon that are holding a Mega Stone.
If that's true, they essentially do all the same things that new Pokemon are intended to do, only without actually being new Pokemon.