I voted for everything apart from 'Berry Trees' and 'Battle Frontier'.
I didn't really use any of them anyway.
Although they could keep it how it is and it would still be perfect!
I have wanted it to be a tournament instead of elite 4 for a while though, like it is in the anime. All opponents should have random pokemon aswell so they aren't always the same.
That's a problem with the elite 4, they are boring to rematch it's always the same, it was random it would be a different battle everytime and would be more fun. Also you would have to think more. =)
Also by a new battle system I assume you mean like new types of battles, like the rotation and double? I wouldn't mind a new one of them, they are fun.
But the way you battle needs to stay the same.