My understanding is that Rock is being saved as a surprise for ‘Mania - with him somehow costing Roman the win on night one, setting up a Rock-Roman match for night two. For which title, I don’t know. I know that Cody would probably prefer the WWE title given the context he referenced regarding his reasoning for a WWE return last year.
It’s also my understanding that if Rock cancels his commitment to the event - they do have a back-up plan - with that being to have Stone Cold Steve Austin do the same plan. Rock is their first choice, but SCSA is being kept off RAW next week to avoid speculation on him possibly returning for another match this year after having had one last year if he is needed. I’m unsure if they will still factor in SCSA in ‘Mania plans if Rock is still committed to WM.