Unpopular opinion but I think Saliba-Mari could probably work quite decently, at least in theory. Both good in the air, Mari probably better with his feet. Saliba can do the running, Mari can do the whole Mertesacker organising the troops thing. If Mari is slow well you've got Tierney next to him who's quick and a good 1v1 defender. Bellerin next to Saliba so even if Bellerin cocks up Saliba could cover with his pace.
You've the whole dog/cat thing with CB partnerships, the dutiful dog and the prickly cat. Mari's the dog, Saliba's the cat, although I think he ends up the dog later in his career. Make sense?
I think it's a big risk going into a season with a guy that has barley kicked a ball in the PL and a 19 y/o kid. From what I've seen as well I'm not a fan of Mari, he is very slow and it doesn't look like he can defend 1v1. Making him permanent was the most baffling decision from the Luiz/Cedric/Mari news imo.
I think Saliba is better on the ball as well tbh.