No way RB puts another outside driver in the seat. Marko even said that Mateschitz threatened to pull funding for RB Junior if Perez gets the seat because all RB drivers are supposed to come in from RBJ (he gave in, obviously).
Sainz - He worked like hell to get out of RB. Even if it was an option, I can’t see him going back to be a second driver behind Max. We already see how much he hates being deprioritized behind Leclerc.
Hulk - He’s a Mercedes Jr, never had backing from Red Bull. I can’t see this happening.
Ricciardo - Maybe for a short stint to cover Perez if he gets dropped. I can’t see him staying long term.