If this was an episode of "Let's Make A Deal" none of the doors WWE has to open for the Universal Title match at SummerSlam would have a donkey wearing a party hat behind it. There was a time WWE thought they had a direction for the Biggest Party Of The Summer, but ever since Samoa Joe was so widely applauded for his program with Brock Lesnar it seems WWE now has another choice on their hands.
Dave Meltzer recently noted in the Wrestling Observer Newsletter that WWE could be looking to turn the Universal Title Match at SummerSlam into a triple threat. However there are still plenty of options to go with at this point.
The #1 contender match for the Universal Title will be a great determining factor on which way they will go. Roman and Joe could have some kind of no-finish and make it a three-way dance at SummerSlam. But then again, Braun Strowman is lurking somewhere and he could always cause some kind of commotion in the landscape for Raw's top title.
Strowman could be inserted into the Universal Title match as well or if WWE felt like saving Roman and Brock for WrestleMania like they originally planned they could always have The Monster Among Men cost Reigns the match. After all, Braun will certainly be seeking revenge against Roman Reigns after he almost murdered him after their match at Great Balls Of Fire.
As unlikely as it might sound, John Cena is a free agent and they didn't make him that for no reason. Big Match John could always be inserted into the Universal Championship picture at a moment's notice if WWE ever feels like they need him. The match might be Roman Reigns vs Brock Lesnar or John Cena vs Roman Reigns at WrestleMania. Then again they could always do something else because Vince McMahon can. But barring any unforeseen circumstances the common factor looks like it's going to be The Big Dog.