Its shiny should also be green. Since Mega Dragonite's shiny is now blue for equivalent exchange and no other reason.You know Garchomp's current mega feels pretty X-shaped
I think a good way to handle the new mega would be to make it kind of y shaped. So maybe you'd like, make the fins more prominent, kind of like wings that jut out a but. And you could demphasize the legs a bit by slimming them down or bringing them in. And make the tail pretty long. but give it a little bob at the end. And since Garchomp "X" is land based, we can take this time to emphasize that it can fly around too, so it should be air borne to really make the silhouette pop. But just making it Dragon/Flying would be weird, so maybe we can give it Levitate!