This is probably a long shot, for many reasons, but worth discussing. Melo in a Spurs uniform would be a sight to behold. Carmelo Anthony has built the reputation of being selfish on offense but losing can change a player's mentality. The Spurs usually only want high character guys on the team and Melo may not be squeaky clean but he is far from a cancer. Popovich has worked with Melo on Team USA for many years and likely could reach the star, getting him to buy into the Spurs way. Melo knows that the clock is ticking on his career, so he should be willing to do whatever it takes to compete for a ring. He is a three-time gold medalist, so he knows what it takes to win the right way and he may be worth the gamble for the Spurs.
The Spurs are in desperate need of perimeter scoring to help Kawhi Leonard and Melo has the skill-set to help with that. Melo does thrive off of isolation plays and schemes but he is fully capable of playing within the Spurs system. He can score in the pick n roll, off the dribble, and capitalize on catch and shoot opportunities. The Spurs system will likely make the game easier for Melo and Kawhi would still be the head of the offense. Imagine having a fourth quarter with both these scorers on the floor at the same time.
Melo is not a defensive player at all and the Spurs will likely pay for that on the court. But the Spurs are already a defensive juggernaut, they need better offense to push them over the top. Melo lacks defensive prowess but he is a better rebounder than people give him credit for. Last season he was even slightly better than Kawhi Leonard, snatching 6 rebounds a game and seven for his career. So he can help the Spurs and provide a punch offensively, but it really comes down to his willingness to sacrifice his individual game to win--which is a big if.