Sarada needs the Rinnegan if she wants to scale up to the stronger enemies in Boruto. She could obtain this power.

For instance, Madara put his Rinnegan inside of Nagato when he was ... Sasuke had already awakened the Eternal Mangekyo Sharingan. However, this wasn't enough to take him down.

Sasuke could finally regain his Rinnegan in Boruto. This power up could allow him to compete with other characters.

Using the power of the Ten-Tails Juubi, extremely powerful wielders like Madara and Kaguya can open the third eye on their forehead to combine the power of the Rinnegan and the Sharingan.