10. Rey Mysterio
In terms of storylines, WWE legend Rey Mysterio has offered absolutely nothing of value to the company since his return a few weeks back. He was bafflingly squashed by Bobby Lashley and has lost multiple matches since.
However, speaking of matches, he has put on two awesome bouts with Andrade over the past few weeks and while, yes, he did come up short in them, it seemed like it was beginning to edge towards to a potential program for the pair - what with Andrade disrespectfully ripping up his mask following one of their matches. It goes without saying that capitalising on that could have been a wise idea.
Now, just because they didn't meet at SummerSlam doesn't mean that the feud won't continue, but it does suggest that it simply wasn't important enough which, given some of the matches that did make the card (and the kickoff), isn't right. Furthermore, as the event was actually surprisingly short for a modern day WWE pay-per-view, there would have been enough room to stick a clash between the pair on it - and, given their previous in-ring work together, this could have been a great addition to the card.
Rey's one of the all-time greats and yet he's been directionless since returning. SummerSlam would have been a nice way to change that while giving fans a match that we all know they would have enjoyed.