Meltzer went on to say that at one point John Cena vs. Roman Reigns had been rumored for WrestleMania after WWE briefly considering having Lesnar defend the WWE Universal Championship against Braun Strowman at SummerSlam. They also considered doing Lesnar vs. Reigns at SummerSlam but ultimately settled on doing a Fatal 4-Way featuring Lesnar vs. Reigns vs. Strowman vs. Samoa Joe. At that point the scheduled WrestleMania main event reverted back to Reigns vs. Lesnar.
It was also noted that WWE started planting seeds for a Roman Reigns vs. John Cena feud on RAW last night. That would suggest that the match between the two will be taking place sooner than WrestleMania.
As for the WWE Universal Championship Lesnar vs. Strowman appears to be the biggest money match for Lesnar right now and that is why WWE confirmed that the match will headline WWE No Mercy next month.