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Megas are introduced as upgraded forms of classic Pokemon.
Fans begin to speculate about which Pokemon will Mega Evolve next.
ORAS announced. A new round of Megas is throughout over 2014. Tons of hype. Speculation continues.
No new Megas in SM, but they coexist alongside Z-Moves. Speed stat for all Megas is also updated.
Megas are then retconned into Kanto for Let's Go. X and Y Mega forms can be chosen on the fly, since there are no held items. Things are looking interesting.
But then SwSh kills Megas AND Z-Moves, establishing a new precedent. Now its "Every game has its own gimmick!", instead of the battle system developing over time.