John Gamefreak decides that megas that aren't Zard and Mewtwo should get a 'X' and 'Y' version because he wants to shake every last shekel out of his consumers. He turns to (You) to create stats/concepts for them! What do you do? Try to make the mega have a separate niche to the other stone - not just an improvement. Try not to make them broken!
REMINDER: If its possible to draw a pattern from the only two variant Megas we have, 'Y' megas tend to be the more 'natural' progression from the base form.
>Zard Y just looks like base Zard but bigger/spikier and keeps higher SpAtk - Mewtwo Y is still a pure psychic Special Attacking nuke
>Zard X gains a dragon subtype, a complete change in colour scheme, and blue flames - Mewtwo X starts taking roids and becomes a physical attacker
Whether the mega we already have for the mon is the 'X' or the 'Y' is up to your discretion.
BONUS ROUND
Masuda calls in, and demands that you create a Mega Charizard and Mewtwo Z with +150 stats. When you ask him why, he replies
>Haha! I don't really know..
and hangs up in order to return to his Lucario fursuit porn.
What stats and design do you give them?
>>32314297
Ash-Greninja becomes Mega-Greninja Y. This is Mega-Greninja X.
>>32314297
Garchomp X loses its scythes, they become similar to jet engines, but spit out sand, and overall appears far more aerodynamic
Becomes ground/flying
>earth power
>hurricane
>new ability - category five
> eliminates entry hazards on switch in, and prevents them from being set
-20 Atk
+20 Def
+60 Spatk
+10 Spdef
+30 Spe
>>32314297
Mega Gyarados Y
>water/dragon
HP: 95
Attack: 125 >95 (-30)
Defense: 79 >109 (+30)
Sp. Atk: 60 >94 (+34)
Sp. Def: 100 > 140 (+40)
Speed: 81 > 107 (+26)
>adaptability
Pretty strong bulky mixed attacker
Becomes thicker, but also more calm in appearance
Mega Sceptile Y
Grass/Dark
Ability: i don't know... Technician? Stakeout?
70
85 > 145 (+60)
65 > 75 (+10)
105 > 110 (+5)
85
120 > 150 (+30)
>>32314507
Actually Defiant, it mirrors Lightningrod in that it's a conditional powerup.