I'd love to add Beal to our roster but not at the cost of Brown. If it was a direct swap, our team wouldn't improve at all. The Celtics would be a plus for Beal's scoring and perhaps passing, but a major minus in defence and a backcourt of Kemba & Beal would be a defensive train wreck. Brown is 3 years Beal's junior, taller and much stronger, thus you can assume his game will age very well in comparison to a 6'3" (something people forget) Bradley Beal.
Opinions vary on Walker (I think he's a negative asset) and I don't want to start that discussion here, but it makes sense that Walker goes out in a trade for Beal, because of defense and salary purposes. So that is a factor in valuing a deal for Beal.
But on a basic level I really don't think the Celtics would be required to offer Brown to get Beal (assuming he wants out to a better team). It's not what fair that's terribly important, but the market. Look at what Houston received for a superstar like Harden. Very underwhelming, but they had to trade him anyway since otherwise they'd be worse off.
Beal has a player option, so he's on a one-year-deal. The Wizard have to trade him
this offseason if Beal doesn't want to extend his contract. And who will be in pursuit? 76ers, Heat, Hawks, Knicks, Raptors, Pacers? Lakers, Mavericks, Clippers, Nuggets, Pelicans, Grizzlies?
I don't think any of these teams are going to offer an All Star in return (expect Simmons, Adebayo, Young, Randle, Siakam, Sabonis, Murray, Ingram to be off the table). A few teams have more picks to offer, but aren't in a great position to compete (examples: Knicks, Pelicans).
The Hawks and Pelicans are the teams who could easily come up with a formidable package. Just not sure these are destinations Beal would be excited about.