First things first. No more, Psychic, Dark, Ghost, Dragon and Fighting Types. Boring. The Elite Four is in desperate need for some diversity. Sinnoh brought us some, with Bug, Ground and Fire typed Elite Four members, and it seemed to be generally well received, so why not try it again? I'd really like to see this generation's Elite Four specialise in Steel, Fire, Electric...even Grass or Water typed Pokemon.
As for the Champion, I really hope they're mixed. Cynthia perfectly exemplifies how powerful a mixed team can be, so I think if we want another real challenge, the developers should invest in creating a Champion with a variety of different Pokemon.
I'm also pondering on the battle themes and appearance of the Elite Four. I want them to look smart, intimidating and powerful, as they should be. I don't want frilly gowns and torn fighting outfits. This also applies to the Champion as well. Every aspect of them, needs to fit the title they play. If the battle music was anything like Unova's then I'm more than looking forward to battling them eventually. If it's some crazy, wonderland music like Iris', then eh...not so much.