Middleton has indicated he wants to stay in Milwaukee, so I bet he gets a pretty big contract from Milwaukee. They will then probably try to negotiate a team-friendly deal with Brogdon because of how much they already have on the books. Depends on Brogdon if he wants to get paid, or if he is willing to sacrifice a little to stay with Milwaukee. Because if they don't come to a deal and he hits restricted free agency, then I think a team can get him if they give him a pretty big offer sheet, one that the Bucks couldn't afford to match.
The question is if other teams think he would be worth the contract it would take for the Bucks not to match it, and if they believe he will continue to develop as well as he has. The Jazz could get him if they wanted if he is out on the market, but they will as with other teams likely overpay to get him away from the Bucks.