Smart is great and all, but he hasn't really shown enough to be the centerpiece in a deal for a star. That's why the Brooklyn pick would be necessary in any deal for him. Right now Smart would still be the nice extra part of the deal, but not the main part. The other thing that's a drag on him is he's always hurt.
Durant is really the guy I'd want even if it's unrealistic. He would be perfect as a 3-4 in our lineup. Plugging in Westbrook would be adding a great talent but it wouldn't solve as many problems with the team that Durant's addition would.
I'm starting to optimistically think that, if the Celtics continue to make noise in the league, they might actually have a shot at convincing that first star to come over in FA or trade (remember, it's hard to deal for an established guy if he says he doesn't want to come, he still usually has to agree) even though we have no other obvious stars on the roster currently.
How about this: Trade for Cousins and sign Durant outright! Then draft Durant's eventual replacement in two years with Brooklyn's pick when they're still horrible!