I've always wanted Kangaskhan to be given a unique ability that allowed the baby to somehow participate, although my idea was automatically giving substitute for 25%HP the moment you switch in. To me, this is the perfect example of what I hope Megas will bring to the metagame: an opportunity for low tier pokemon, in this case a "PU" pokemon, that need a boost in battle yet either can't evolve further (Ampharos) or already seem in appearance like they're fully evolved (Absol).
We haven't even seen the 6th generation metagame, so we can't call anything broken yet. This reminds me of back when people were saying things like no-weakness Spiritomb, Prankster Whimsicott, Zoroark, and Magic Bounce Espeon were going to be OP without any futher knowledge.
But let's say hypothetically Mega Kangaskhan was in Black/White OU. She's only 90 base speed,
tied with Lucario, so quite a lot can outspeed and revenge kill her.You
cannot hold choice scarf or salac berry because you're already holding a mega stone, not to mention you're unable to hold any other strength-boosting item. I imagine that 6th gen will be even more offensively paced than the last generation, so relatively speaking, even more pokemon will be able to counter her.
Sorry to all those who hate overly competitive talk, but I don't think a person can really claim that something's broken without considering things more broadly than just "we've never seen this before."