It depends on the type of sound he's going for but here's some ideas:
OutKast or at least one of the members, since he has done collaborations with UGK and a member of Three-6. Project Pat would be dope too.
Lorde or Billie or some other alternative artist. He has some dope features with LDR and Selena Gomez so I think it could work out well.
J. Cole would be ill - just so he can say he worked with Kendrick, Drake, Kanye, Tyler, Future AND Cole?? All of the 2010s GOATs.
More classic legends would be dope for obvious reasons. Nas, Eminem, Jay-Z, a Wu-Tang member etc. would just be cool for the culture. I could picture a more traditional boom bap 1Train-esque song with one of them on plus Ferg. (We know he can carry his own too since we had that song with Mos Def).
