In order to move up for Barnes my guess would be a team like Portland, Sac, or GS would want Pierce. All three teams have pieces in place to make the playoffs next season they just need leadership/scoring at the 3.
example would be
Paul + 21+22 to GS for #7 and #30
GS becomes a legit playoff team with Curry,Thompson,Pierce,Lee and Bogut and paul goes home to California
I personally would have a hard time with this trade but can see why it would happen.
Im not for using Rondo or AB to trade up.