Both the matches between John Cena vs. Roman Reigns and Braun Strowman vs. Brock Lesnar have been scheduled for a really long time, but due to timing, injuries and other unforeseen situations and circumstances, the dates were never finalized and the matches never pushed through until No Mercy later this month.
Due to the sheer significance and importance of who the superstars are as well as the magnitude of their matches, the feeling is that WWE will try to get multiple matches out of each of these rivalries, but ultimately it would depend on the plans WWE would have for WrestleMania. As of this time, the ultimate plan is to build towards Roman Reigns being WWE’s top superstar.
While there is no official word yet, the likely plan for the match between John Cena and Roman Reigns match is to get Reigns over. But as of the moment, the plan is for Roman Reigns to lose the match. It is being said that his loss to Cena would only happen if the company feels that a loss would be more effective for him to get over from a long term perspective.

Source: Wrestling Observer Newsletter