#1 Elias is disqualified
When Elias initially challenged Reigns to a match at Money in the Bank, it was during a backstage song. Elias isn't shy about whacking anyone over the head or back with his guitar, so a loss by DQ is a possibility for the match at MITB, albeit a disappointing one.
Any time the two foes have crossed paths, it has usually ended in chaos. Whether someone is caught interfering on his behalf or he resorts to using his faithful guitar to defeat Reigns, Elias could easily be disqualified.
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Elias losing would give him a reason to continually target 'the Big Dog' as he would not be satisfied with losing. Even if he does blatantly cheat by hitting Reigns with his guitar, the ref might even be coerced into letting it go.
If things are on the level in the match (and when have they been in this feud) then 'the Drifter' might resort to cheating when all else fails. If he's caught and a McMahon doesn't let it go, then he could easily lose via DQ.