I think they would want to re-sign him--and would want some assurance they could.
I also think they should check with Al Horford's sister before they make a serious offer. Or Al, himself. I though they blended well in Atlanta, but Al may not like his center role in Boston. Whatever, Millsap can play the 4 or 5. He's one of the best. A max player they might be able to squeeze under the cap with IT eventually if they unload a player in a deal.
In fact, a deal with Atlanta could be an opportunity to make cap room for both IT and Millsap. So maybe keeping the Nets pick makes sense if they trade somebody affecting the cap.Smart's salary isn't high enough affect the cap space. It would have to be expiring contracts, and I wonder if Atlanta would be interested in any of those players: KO, Zeller, AJ. Doubt it. They'd have to throw in a draft pick or one core player.