Defensive Mega Pokemon* and Mega Charizard* are going to be useless
* assuming the mega evolution doesn't occur on switching in. Over the past the 5 generations walls have been at full defensive capabilities from turn one considering the incoming attacking attack its switching into and the entry hazards present (when was the last time you saw a competitive cosmic power wall or a wall weak to stealth rock). Mawile doesn't have the stats to take any hits neutral or otherwise, Mega mawile on the other hand can, but it won't be switching into the attack 60/85/60 mawile will be instead .Revenging with him, is a no-no its speed will barely change . labouring the Mawile scenario more, assume the mawile survives the switch in and hazards and takes 50% of its health and evolves next turn. What can a decent wall with no recovery and no leftovers wall now? It might be to dish out a super effective hit and possibly KO (assuming the player didn't pay attention to the team preview or was smart) but it won't take much effort for the opponent to counter. Gardeivoir might fare better, having better defenisive stats but its low speed and lack of recovery will hamper its bulk and its effectiveness. Or alternatively you could have a wall that doesn't need to mega evolve and has full walling capability from the second it hits the field. Amphy its the bulkiest ( and is good pokemon I've seen it used effectively in gen 4) and could possibly have a chance to have 60%+ hp after evolving and have a few swings with 180 base.
Charizard will still suffer from stealth rock and its bad typing, like the past few generations. Its power could equal Salamences' so why not just Salamence that doesn't have the problem with stealth rock and bad typing.
Mega Charizard X will be perfectly fine. It's a Fire-type neutral to Water and it can Mega Evolve, outspeed any Water-type lead Rain has, and throw out a Sunny Day or a Flare Blitz on the same turn. Politoed can do jack to Charizard X without a Choice Scarf and Ferrothorn just gets nuked by Flare Blitz. It's really the one Pokemon that Sun teams have that absolutely dominates Rain teams. Sun teams should have a way of dealing with SR from the start anyhow, so it shouldn't be that much of an issue. Honestly, just evolve Zard early and you're golden.
I can't say the same for Charizard Y as it retains it's Water weakness, and just becomes a stronger but frailer Ninetales even with Sunny Day.
All this, i've yet to speculate on how many times you can have a pokemon evolve into its mega form in one battle, if its the one time. Why would anyone want a mediocre defensive poke on their team, surely having a equally mediocre sweeper like kangaskan or lucario.
Welcome everybody to Gen 6, hyper offensive is the game and the only way to play.
Have you even looked at some of the new Pokemon?
More than half the Kalos Pokemon seem to be slowish, defensive behemoths with 120~150 in one of its defensive stats or tank.
Goodra is every Grass-type's worst nightmare, Kelfki is a Steel/Fairy-type with Prankster of all things, Doublade and Aegislash abuse Eviolite and Stance Change respectively for insane bulk, 3 families of Pokemon get Frisk so item bluffs fail, Two Grass/Ghosts for Keldeo, Florges as a Fairy with Base 150 Sp.Def and Recover, and Zygarde is built to be a faster, bulkier, but less powerful Garchomp. There are even better forms of Protect that damage/cripple the opponent if they attack physically, Electric-type and Fairy-type Giga Drain variants, and a large handful of Attack/Sp.Atk-lowering moves that either do damage, have priority, or switch-out the user ala baton pass.
Fairy also completely ruined the Choiced Dragons and Fighting-types that made Hyper Offense what it was and they can't be removed via Magnezone nor Pursuit. Fairy will effectively
bury 5th Gen hyper offense teams. For instance:
Meet Granbull, a fairly mediocre Pokemon. He's a Fairy/Normal-type that also has Intimidate.
Granbull Intimidates then crushes Dragons with Ice Punch/Fairy STAB and Gengar with Crunch. It can even revenge KO Terrakion with Close Combat.
Now take something like Sylveon or Florges with Wish/Recover, high Sp.Def, and Sp.Atk and hyper offense is doing no damage without some powerful secondary STAB or Steel-type attack on those Dragons.
For every Dragon that carries a Poison or Steel move, they miss out on boosting moves or on threatening Heatran, Ferrothorn, Forretress, or Skarmory with Fire Punch/Blast or Earthquake.
Defensive teams have a very real chance of making a comeback with all the tools Gen 6 has given them while hyper offense seems like it will be much less effective because of those same tools.
Honestly, if weather does indeed get nerfed in a significant way, there's not much more I could ask of this generation.