Let me try to respond to each of these...
TMs being reusable: WHY??? Part of what made TMs unique was that they could only be used ONCE. It kept down the amount of spam in competative - your opponent couldn't have five Earthquake sweepers, so your Pokemon with TMs had to be well guarded or able to take hits or dish damage effectively.
Triple Battles/Rotation Battles: Triple Battles were actually kind of neat, but I think the appropriate place for them would have been to use them when you fight the bad guys. When you are dealing with criminals who want to kill literally everyone, there is no justification for facing your opponent one-on-one instead of sending your whole team against them. Triple Battles could have been used in this way. Rotation Battles are just wierd.
Megas/Powercreep: First of all, they have altered base stats in the past. Second of all, Megas are designed to fill niches and add a new level of play. Megas are designed to take Pokemon and expand on their strengths. Blaziken (which was already OP), simply gains more stopping power from his Mega. In Ubers tier, he needs it, as many teams simply wall away Blaziken with Giratina or Kyogre, rendering Blaziken nearly useless unless those two can be killed off. His Mega allows him to wallbreak more efficiently. The key with Megas is that every Mega puts the user into a niche, where as the base Pokemon are more flexible. Mega Swampert for example is looking like it will be a fast physical sweeper (one that will be VERY efficient at its job), but it's not going to be as useful in its utility role. Base Swampert can go Offensive Support, Offensive Pivot, Defensive Assault, Full-on Assault, or Offensive Utility.
Megas for Pokemon that otherwise don't evolve: The reason for this (and actually the reason for some of the Megas for evolved Pokemon as well) was to give not-widely-used Pokemon a niche to work on a team. I can't recall ANYONE using absol before X/Y, but Mega Absol changed things so that Absol was now much more viable. It can actually dish decent damage now, and Magic Bounce fouls up anyone trying to run a Prankster set.
Sky Battles: I liked them, could be expanded on. Essentially it's a way to introduce people to the concept of Monotype teams, a competitive bracket that gets unfortunately little attention. I would like to see versions for the other types as well.
Horde Battles: Right now they're XP buffets, but I could see them working as a way of adding a sense of dynamics to the Pokemon world. One of the things the Pokedex always mentions for example are the occasional natural predator. In one set of Horde battles for example, and group of four Seviper will gang up on a Zangoose, and will actually kill it and ignore you, robbing you the chance for a rare catch (The numbers are swapped depending on the version). I would love to see more things like this - say a Swellow attacking a bunch of Wurmpule.