Ferrari? Nope.
Mercedes? Nope.
McLaren? Unlikely.
Renault? Possible, but unlikely.
So what other choice does Ricciardo have other than the above?
Only one, Red Bull.
Yep, I cannot
realistically see DR with any other team other than Red Bull in 2019. I think he waited too long to make a definitive decision. The rumoured pre-contract with Ferrari runs out in a few days. And as part of that apparent contract, he was to not talk to any other team. So I doubt he is on either of the top two teams radar now. DR will once again be playing second fiddle to Verstappen in 2019, because RB have openly admitted that they'll do whatever it takes for MV to be the youngest WDC.
But I could be wrong. It has happened in the past.