I agree with PK Gaming, old BW Sun teams are dead, as a matter fact, rain also is due to the sheer amount of weather abusers generic rain teams have. Eh, on the subject of Mega Zard Y, i find him as a destructive force with nothing to switch into bar: blissey, chansey, specially defensive politoed, and, well thats it.
Mega Zard Y can fill in a space that ninetales cannot: a wallbreaker, literally no offensive team can switch into a drought boosted 150+ (i dont know the exact stats for Charizard Y) Fire Blast, you can then kill him (which is probably going to cost more than we're giving tha bulk credit for), and then a simple non-set up chlrophyll sweeper can come in to wreck havoc. This is obviously assuming no rocks are up and Charizard comes in on something threatened by it, but you get the idea, i think charizard Y, even with the rocks weakness, can bring easy momentum to the table, something ninetales cannot do consistently and we know it, and create a new archetype for sun teams this generation.
CTC umm, the OP said Mega Gengar might be viable, and that Mega Blaziken is meh, this, btch plz.