If I'm going after "future Paul Pierce" then I'm going Jimmy Butler. Hayward you can make a really good case for, but Butler is better in two key ways:
1. He's on a MUCH cheaper contract for at least the next three years (he'll opt out when his player option hits, as he should) then any contract Hayward would be on after 2016 (he'll opt out and sign for mega money).
2. He's more accomplished as a star -- he plays defense, offense, everything, and he can do it as the alpha, which he wants to be. Al's a great guy but as an alpha player, he's never been quite good enough. He's a better beta than an alpha, IMO (and I'm a Gators fan, I saw him win back-to-back titles, but he never was "the guy" on those Gator teams).
Given that Chicago is a mess right now -- they signed Rondo and Wade (why? Who knows) and Butler is probably tired of their crap. The Bulls will want cheap, young players back and guys they won't have to pay a ton to in the coming years -- they'll want to sell their fans on Wade/Rondo in the now and the young guys/picks as the future.
So, I'd send Brown + 1 future first (not Nets -- those are gold/nuclear weapons, only use to nab someone who's SUPER good or going to be). Toss in a young guard (Rozier/Young ... probably not Smart as seeing he'll be wanting lots of money after this season and the Bulls probably wouldn't like that commitment). Maybe a definite low future, future 1st if it feels unfair.
It's a steep-ish price, but Butler is worth it and can instantly catapult you into title contender status. Him, Horford, and "future" Ray will be special.
I'd also ship out Isiah Thomas. I like his game but he's too ball dominant a scorer and not as accomplished a passer as Rondo was for that Celtics title team. I'd move Smart into the starting role, knowing he won't bring much offense (Rondo didn't either) but he'll lock down dudes. Defensively, your team will be nasty with a lineup Smart, Butler, and Horford in the lineup. I'd use Thomas to grad Future Ray or bolster my bench with talented youngish bets.
The scoring order in the first-unit would run Butler/Horford/Future Ray, and anyone else is gravy. Get a scorer for your second-unit to bolster the bench, a good two-guard (not undersized) and you can flip some lineups around if the situation needs it. Butler can be a two or a three, he's more naturally a two, but his defensive chops may be needed at the three for guys like LeBron.