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The Gen IV starters, dude. Specifically Torterra. I feel like they've gotten little to no love, with the last time we've seen Sinnoh is about to be going on 9 years now. Such a shame, considering they're all really solid typings and each can fill their designated role with excellence. They desperately need some form of buff or boost to make them relative again, and I feel like we're gonna get that in the form of an exclusive z move or a new mega during the inevitable Gen IV remake.
Now, specifics. Torterra is probably one of the least loved grass starters, above Meganium and Chesnaught, but below Sceptile. While his speed is shit, his beefiness and great physical stats more than make up for it. If he had a Mega, I'd personally want it to be allocated to his defense, sp. defense and attack. Just make him a tanky but slow powerhouse.
Empoleon, from what I've seen on /vp/ and others, is probably the most loved out of the three. Once again, kinda shit speed. Essentially Torterra but with physical and special stats switched around. I've never really been fond of it, but if it had a mega that added 30 speed, 50 special attack, and and 20 hp, I would definitely give it another look.
Infernape was, I'm guessing, supposed to be everyone's favourite, the Charizard or Blaziken of the region. He takes away some of Blaziken's attack and hp, and just shoves it straight into speed. That's not really a bad thing, I suppose I can see what Nintendo was trying to do with it. For a mega, he'd definitely need an attack boost, and maybe a little bit for each of his defenses too, to make sure he doesn't drop the moment a water or psychic type comes around.
Either that, or they can all get z-moves. Which is probably 10x more likely.