I doubt Aldridge and Gasol leave their teams. Monroe, on the other hand specifically signed the Piston's qualifying offer so he can be a restricted FA this offseason. He is the most realistic possibility but I don't know if Pop will want him. Monroe's still developing his jumper and isn't that great on defense. He is only 24 though so there is still time to develop. But as good as he can get, he'll never anchor a team's defense like Duncan or Gasol.
Gasol is the best fit but he has ties to Memphis and the Grizzlies might be in a better position than the Spurs if Duncan and Ginobili retire next year.
Spurs still have to clear the hurdle that not many FA wants to play in San Antonio. Probably better to just tank and build through a lottery draft pick like they did with Leonard, Duncan, and Robinson.