2. Kevin Owens and Sami Zayn

Kevin Owens and Sami Zayn are currently out due to injuries. Owens is recovering from two knee surgeries while Zayn is recuperating from a shoulder injury. They are still both members of Monday Night Raw, and both are rumoured to be surprise entrants in the 2019 Royal Rumble Match, per Cageside Seats.
When they were on SmackDown Live, Owens and Zayn had a feud with Shane McMahon and Daniel Bryan. Even though they lost that feud, they could have used it to propel themselves as tag team contenders. But before they can do it, their injuries happened.
Once Owens and Zayn return, the WWE should use them to re-establish the tag team division of Raw. They are two of the best performers on the roster, and Zayn needs to win his first title on the main roster. Even Nicolas has more title reigns on the main roster than him.
The WWE could even go further with Owens and Zayn battling for singles titles as tag team champions. Owens challenging for the Universal Championship while Zayn goes for the Intercontinental title. It might not happen but things should be looking up for the real-life best friends.