I personally want to see Elite 4 trainers to have strategies, not just type themed like they're buffed up gym leaders. Based on what we have so far, this is personally what I want to see.
Battle 1: Just to put pressure on you, this dude dishes out fast attacks that deal tons of damage. With moves that prevent you from using items, you'll have to fight back equally as hard to survive. If this guy was in the 4th gen, he would have Pokémon like Salamence, Metagross, Rampardos, etc.
Battle 2: A Baton Pass team filled with Pokémon with the Sturdy ability (or a focus sash) with the end result being a powerful mixed sweeper.
Battle 3: A special kind of battle. Any Pokémon that is not knocked out faces another Pokémon of its own type in a 1 on 1 match. You cannot switch or use items at this time. At the end, if you manage to win more battles than the elite 4 member, then you win.
Battle 4: A general annoyance team. Includes Pokémon with access to many annoying moves like Trick, Thunder Wave, Torment (after being Trick'd a Choice Scarf), etc.
Battle 5: Each Pokémon on the champion's team has some sort of way to cover one of the other teammates. Altogether, his 6 Pokémon's types end up at least resisting every single type in the game.
It won't happen, but that's what I want.