WrestlingNews.co has been told that The Rock is still not locked in for WrestleMania and the current plan, if The Rock can't do the show, is for Cody Rhodes to challenge Roman Reigns for the Universal Championship. WWE wants to get The Rock committed before the Royal Rumble because that is when things will get set in motion for Mania.
WrestlingNews.co is told that this is the current plan for the main event matches for WrestleMania:
Seth Rollins vs. Drew McIntyre or Cody Rhodes
Roman Reigns vs. TBD. If The Rock can't do the show then the plan is for Cody Rhodes to take the spot
It's not clear yet how the company will split the WWE Championship and Universal Championship but the idea is for each brand to have a top title heading into the summer.