#2 The Club
Luke Gallows and Karl Anderson made their debut on RAW, as the Superstar ShakeUp saw them making their way back to the RAW roster.
Another Superstar who is also on RAW now is AJ Styles.
A heel turn by AJ Styles would be perfect for him to reform The Club on WWE RAW, and this faction could end up dominating the roster. It would also be perfect, as it would likely stop the rumours of Gallows and Anderson leaving WWE to join AEW.
#1 The Undisputed Era
Let's be honest, this is something everyone wants to see.
With The Shield gone from WWE, The Undisputed Era is one of the top factions on the roster by far. While they are going through difficulties of late, and they are on NXT, they are the only faction currently existing who could do fill in the shoes of The Hounds of Justice.
Adam Cole, Bobby Fish, Kyle O'Reilly, and Roderick Strong might be the future of WWE if given the proper push, and when they make their way to the main roster, that is exactly what might be waiting for them.