The main event plan for WrestleMania 40 is looking like the Cody vs. Reigns rematch, but depending on how the coming months play out Roman's opponent might actually be LA Knight. From what I've Cody would be fine with it as long as he either gets a World Heavyweight title win or going over a massive star at Mania and being next in line for the Universal title if Knight's time comes.
Also as for the question if The Rock is ever going to return for WreslteMania? No, they've given up. Last year was their best chance but with Black Adam failing and Dwayne having a mental breakdown, their chances now are slim to none. Remember they cancelled the Acknowledgement Ceremony that was supposed to be a vehicle for his return, but as soon as he ran off into the woods and calling himself a t-rex they scrapped it for The Trial of Sami Uso.