#2 No other exciting challengers for Seth Rollins at the moment

It would be a long shot to assume that Braun Strowman is going to win the WWE Universal Championship at Clash of Champions 2019. As we've seen over the past couple of years, whenever Braun Strowman has a lot of momentum and a title shot at the same time, the entire purpose of it is to benefit the other Superstar.
Two years ago, it was Brock Lesnar. Prior to WrestleMania 34, it was Roman Reigns and this year, it's probably going to be Seth Rollins.
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Given that he's just off the cusp of a clean victory over The Beast Incarnate Brock Lesnar, it doesn't make sense to put the title on Strowman, even if they want to run a Strowman vs Wyatt match.
Either way, the fact of the matter is that post-Strowman, there aren't too many good challengers for the title. AJ Styles is busy with the United States Championship while other contenders like Drew McIntyre don't really have the momentum behind them going.
If we're talking purely in terms of having momentum, then no other superstar is more exciting for Rollins to face than The Fiend.