Why throw out all these popular designs that get used in marketing a shitton after one gen? They may throw the game balance out of wack but that's not reason enough to trash the designs entirely.

Instead, I bet you they're just gonna have special Z-moves tied to Megastones that essentially work like Rayquaza's Dragon Ascent, but without the 100 BST increase, while keeping the stat balances, typing and abilities in-tact. Pokemon without extreme changes in their Mega-forms will just use a powerful attack and keep their mega-appearance for the rest of battle/
Extreme example:
>attach Charzium X to Charizard
>send it into battle, have it use the Charzium X Dragon-type Z-Move 'Wyrmbreath'
>in the attack animation, it transforms into Charizard X and attacks
>after the attack for the rest of the match, it becomes Fire/Dragon type with Tough Claws, and it's Atk/Sp.Atk are evened out
Regular Example:
>attach Gardelium to Gardevoir
>send it into battle, have it use the Gardelium Fairy-type Z-move 'Waifubait'
>it transforms to Megavoir, does a powerful attack, whatever
>no other changes

>this will be the case for every Megaform