#3 Reigns gets disqualified in some fashion
Since Shane McMahon has been heavily featured in this feud, he will likely play some factor into the outcome of this match. If Reigns and Elias face off before the steel cage match between McMahon and the MIz, then Shane will most certainly interfere in some way.
He might come out to ringside during the match and constantly taunt Reigns at every turn. Another thing he could do is find the smallest of reasons to have the ref DQ Reigns. Shane McMahon's abuse of his power has also been on display during his feud with Reigns and the Miz.
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It will come into play somehow on Sunday and could reasonably affect the outcome of several matches at the PPV.
Daniel Bryan and Rowan are defending the SmackDown Tag Team titles as well, so along with his own feud with The Miz, we might experience a power-drunk McMahon in the highest fashion at MITB.
If a person in power somehow affects the outcome of the match, it will also give Reigns a good reason to continue opposing the McMahons.
It might be another way the WWE tries to keep him over with the fans because even though he's currently cheered, the more he shows up everywhere, the quicker his good will with a lot of the fans will fade.