Unless Butch and Cassidy become like they originally were before Chronicles, I wouldn't enjoy seeing them more often than we already do. I preferred them when they were like the anti-Jessie and James, which basically meant that they were more threatening and more competent, despite the fact that they always lost too. Ever since Chronicles, they've felt more like carbon copies of Jessie and James, only without their chemistry and humor.
I still think that there are a few Brock fans out there. I still like him and I'm also not in favor of replacing him. Sure, he doesn't do that much, but my enjoyment of the series isn't damaged by his presence either.
I've heard about that statement about Brock looking to the future with some kind of goal. While I agree that Brock leaving is a possibility, that statement could also refer to a role he'll have in the new region or something special about the new region itself. They left it fairly vague, so I don't think it's a guarantee that he'll leave based on that.
I wouldn't mind if they did have four people in the group again. It could be Ash, Brock, new girl and a new boy. I doubt that they would have the new male trainer in the group considering that they've never made a big deal out of them before. One of the aspects of AG that I did like was the main cast being larger than before, so it would be be cool to see that again.
That would be an interesting approach. I'm not sure how that would work since she would have other Pokemon and the writers would want her team to consist of mainly fifth generation related Pokemon.