Backstage reports indicate that the long term plan is still to have Roman Reigns and Brock Lesnar headline WrestleMania 34. That match was actually being considered to be moved up to take place at SummerSlam. 
However, when the match between Samoa Joe and Brock Lesnar did much better than expected from an audience interest perspective, the decision was made to change the match at SummerSlam to a Fatal Four-Way. It still remains to be seen whether the match between Brock Lesnar and Roman Reigns is the direction that WWE will take leading up to WrestleMania 34.
Source: Wrestling Observer Newsletter