#3 A completely unexpected returning challenger
As demonstrated by the crowd response to Kevin Owens and Mustafa Ali at Fastlane, the fans want Kofi Kingston challenging for the WWE Championship and are not going to be satisfied with anyone else—even stars they ordinarily like and may have chosen over Kingston a couple months ago. Owens, Ali, a returning Sami Zayn, or even Dean Ambrose deciding to stay with WWE and go for the title—none of these routes that would have, on paper, worked at the turn of the year would be well received at this point in time.
If WWE did decide against Kingston, it would have to be an unexpected, and truly electric wrestler to ably fill his spot on the WWE Championship match. A big call up like Johnny Gargano (NXT Champion vs. WWE Champion?) might satisfy hardcore fans to a degree. A legend like Shawn Michaels, The Undertaker, or John Cena would have promise, particularly for the casual fan base. The Rock is probably the biggest name WWE could plug into this spot. A returning CM Punk may well have buzz enough, though he carries his own baggage (besides which, there’s no indication he’s open to a WWE return).
In the end, if WWE were to choose against Kingston, they’d surely be shrewd enough to at least throw in a major curveball. Nonetheless, it’s hard to imagine fans being happy with anyone over Kingston at this point.