Regarding The Rock vs. WWE Universal Champion Roman Reigns possibly happening, The Wrestling Observer Newsletter reports that Rock would love to put his cousin over at a WrestleMania event in front of a big crowd, but the issue has been insurance-related as far as other work Rock has coming up. The insurance issue has been a problem since the second WrestleMania match with John Cena at WrestleMania 29 in 2013.
Paul Heyman noted in a recent interview that Rock was the one who brought up a potential match with Reigns.
WWE has had at least three ideas for The Rock at WrestleMania events, but none have ever materialized. Those ideas were a singles match against Triple H, a tag team match with Ronda Rousey vs. Triple H and Stephanie McMahon, and a singles match against Brock Lesnar.