Mega Charizard Y kept its Fire/Flying-type, but I'm hoping we might get Fire/Dragon on Mega Charizard X. It will not only take neutral damage from Fairy-types but it also be only 2x weak to Stealth Rock in comparison to 4x. I bet its flames will be extremely hot given that blue flames are very high temperature.
But still, I'm hoping that Venusaur and Blastoise get an alternative Mega Evolution that's version-exclusive as well. It'd make sense not to leave them out. I was even surprised this thing wasn't a shiny and is actually black-colored. Definitely a very drastic change in appearance compared to the other Mega Evolutions we've seen so far.