3. P.J. Washington
P.J. Washington re-signed with the Charlotte Hornets to a three-year, $48 million deal and was a restricted free agent before doing so. The Mavericks couldn't have paid him this much but could have sign and traded for him.
Despite Dallas showing no interest in pursuing him, this would have been a move that would have made Dallas Finals contenders. Last season for the Hornets, Washington averaged 15.7 points, 4.9 rebounds, 2.4 assists, 0.9 steals, and 1.1 blocks per game.
Washington would give the Mavericks much-needed positional size, and his defensive versatility would fit great next to Grant Williams. Both players can switch on the perimeter, and they would give Dallas the best defensive frontcourt they'd ever had next to Luka Doncic.
The Mavericks starting five would consist of Doncic, Kyrie Irving, Grant Williams, P.J. Washington, and a starting center (likely Dwight Powell). Williams and Washington paired together would be an excellent defensive frontcourt and complement each other's games.
This move would be a step in the right direction to get the Mavs back to playing good defense, and they would have landed an experienced forward who can even play some small-ball center if needed.
Although both players primarily play power forward, the Mavs would have found a way to make this work. Washington is also only 25 years old and would be a great piece for the Mavericks' young core.