#3 Should happen at a bigger stage

While I agree that the whole idea of "Hell in a Cell" fits pretty well with supernatural characters like The Undertaker, Kane, and now The Fiend, the coronation of a demonic entity as the World/Universal Champion should take place at a bigger stage than this.
No matter how good the idea of the PPV is, it is still not considered as a top-show. Having Wyatt claim the top spot at an event like Royal Rumble, WrestleMania, or even Survivor Series would create bigger shockwaves.
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#2 Might hurt his momentum when he loses it
As much as Bray Wyatt deserves a lengthy reign with the title for all the effort he has put into this character, it's tough to see him getting a Lesnar-level 500+ day reign. If WWE decides to go ahead with this match at Hell in a Cell, there is no way The Fiend is walking out of the arena without the title, no matter who his opponent is.
But remember, he can't hold the title forever. At the very maximum, he might keep the title till WrestleMania. But would you want your biggest Superstar to lose his first match at WrestleMania as the Fiend?
Rather than that, a better way would be to build his character and let him remain undefeated for some time before thrusting him into the title scene.