I think Tsunoda isn't quite there yet, he's still too rash in certain moments. He has improved a lot, but he still has some way to go before he can get a seat at RBR.
I also genuinely doubt if Checo will be out of the seat. Sure, he's way behind Max, but that would also be for his replacement (as long as it isn't Lewis or Fernando, but they won't sign at RBR I think). RBR is comfortably leading both championships, and they don't have to manage an intra-team battle. Must be nice being them at the moment.
But if they do replace him, I'm putting my money on Hulkenberg. He was also the other serious centender for the seat when they picked up Perez. He is driving very well in the Haas. He will know his place in the team. And to be honest, I just want to see him on the podium one day...