#4. Roman Reigns and Paul Heyman break up, but their roles don't necessarily change
It's possible that WWE will simply split Roman Reigns and Paul Heyman due to some storyline tension between the two. They might not get a full-fledged payoff with this approach. That won't necessarily be a bad thing if their roles don't change.
Reigns could still be the version of himself that fans are currently seeing, and Heyman could go back to being the character he had been portraying on WWE programming. This direction would be a relatively simple, albeit inconclusive, way to end their partnership.
#3. WWE decides to quietly end the alliance
While this is an unlikely result, it's also a safe one, both for Roman Reigns and Paul Heyman. The quickest and easiest solution would be to simply have the alliance between Reigns and Heyman quietly end.
In this scenario, Heyman would simply stop appearing alongside Reigns, and both men would move on. WWE would likely provide no explanation of the development. This progression would be unsatisfying, but because Reigns doesn't necessarily need Heyman, they could just abruptly drift apart.