The idea of giving up either Brooklyn pick for Millsap seems absurd to me. Yes, Millsap is a legitimately good player, but he's not the difference between winning or losing a playoff series against Cleveland. The only way I move one of the two remaining Brooklyn picks is as part of a package for a true star (Butler, George, Brow, etc.). With or without Millsap, Cleveland is still going to have the best three players on the court. Granted, IT4, Horford and Bradley are good enough players where they'll have stretches of being the 2nd or 3rd best players, but not for 48mpg over a seven game playoff series once the Cavs turn up the intensity.
I have no problem with moving some of the surplus assets to get incrementally better and improve the chances of getting to the ECF, but to move a Brooklyn pick for a few months of a (quality) supporting/complimentary piece, no way. Barring unforeseen events, that's going to be a top 3-4 pick again this year. I can't see any path to Brooklyn being much better next year either.