According to Dave Meltzer on Wrestling Observer Radio, there is news going around that Roman Reigns could possibly defeat Braun Strowman at the Great Balls of Fire PPV and then go on to face John Cena at SummerSlam.
This would make more sense now since Cena is advertised as a free agent. As we all know by know Cena will be floating between both Raw and Smackdown. This match is strongly being considered since WWE wants to make SummerSlam as huge as possible.
The other scenario is that Braun Strowman would go over Roman Reigns at Great Balls of Fire and he will end up being the new #1 contender and face Brock Lesnar as originally scheduled before his injury.

Source: Wrestling Observer Radio