Best for them to round up the team with quality role players, they already have a potential all star in Kuzma as the third cog in their superstar triumvirate. Adding another superstar with an ego to manage can be an expensive liability in the long run (the likes of Irving or Butler).
They need shooters and more shooters plus a defensive PG or wing. They already have a solid front court. Now the likes of Brook Lopez, JJ Redick, Danny Green and a Ricky Rubio makes more sense than getting 1 more superstar.