Ruiz has a unique size and shape to him. He's 6'0 (no way is he 6'2), short arms and weighs 260 lbs with lightning fast hands. I think Joshua should try and spar 6'0, fast cruiserweights and just try and box them.
He was sparring Joey Dawejko until he got knocked down in sparring. He should spar fast fighters in general and learn to keep his distance instead of fighting in the pocket.
Joseph Parker, who was able to nullify Joshua's right-hand; for example. Hughie Fury? Possibly take the tactic that Dominic used in preparation for himself, ironically, by sparring cruiserweights.
I don't think there are many heavyweight or cruiserweight fighters that can closely emulate Ruiz's style to sufficiently gain an advantage in the rematch. Development will depend on Joshua's mastering his innermost fears . If only Sigmund Freud were alive.
