I think this is what it will take to get it done to just trade him into our cap space, and if he would extend-and-trade with us, you absolutely have to do it.
They should have no interest in anyone like Bradley or Crowder, because if they think GS will be the dominant force that many think, there's no point in fielding a competitive team right now. They should get as many young, high-ceiling assets as they can for him, along with likely a top-5 pick that they can pair with their own high pick in next year's strong draft. That'd leave them with a core of Smart, Oladipo, Brown, Sabonis, and Adams, which is actually a ridiculously strong core to go forward with along with the picks they'd get.
Also, if any team values Smart, it's OKC. That would take him back to his roots in the Texas/OKC area, where he's played ridiculously well the last two seasons:
Smart 2015-2016 @ OKC: 26 pts, 8 rebs, 3 asts, 1 blk, 1 stl, 9-14 fg, 3-5 3pfg, 5-8 FT
Smart 2014-2015 @ OKC: 25 pts, 9 rebs, 5 asts, 2 blk, 2 stl, 8-14 fg, 7-12 3pfg, 2-4 FT
And even if he wouldn't extend, you almost have to take that chance and do it anyways. Just think, if he didn't extend, then we'd have his Bird rights for next year and could add someone like Griffin before ultimately re-signing him.
But if he would extend and be traded for this package, that'd be the best-case scenario and would leave us with the following roster:
PG: Westbrook, IT
SG: Bradley, Rozier
SF: Crowder, Free Agent
PF: Olynyk, Mickey/Guerschon
C: Horford, Zizic
That's a championship roster and one that could compete with both Cleveland and Golden State.