#3. Aleister Black
One of the problems we have with adding Aleister Black on this list is that it exposes a clear fact about Monday Night RAW - that there is a lack of top heel challengers for the WWE Championship. This could have been one of the reasons why Ziggler was brought over from SmackDown to challenge Drew McIntyre (also added to the fact that they have a history).
Aleister Black seems to be on a hiatus, but before he was, the Dutch Destroyer was among the most consistently-pushed WWE superstars on the roster. He wouldn't necessarily have to beat Drew McIntyre, but it could have made for an intense face vs face feud.
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Of course, one could argue that it's far too soon to put Aleister Black in the WWE Championship picture, but since he doesn't have an active streak as he did in NXT, it wouldn't do too much harm for him to lose to Drew McIntyre as long as the two would deliver in a match.