If Naruto were to have another spell, what would it be?

For me, I'd say probably the Shadow Imitation technique. He can have some of his shadow replications cast it, catch the enemy, then use Rasengan.

Btw, it is proven that shadow replications can use ninjutsu. You can see Kakashi's shadow replication use the Sharingan and the shark missile technique when he just arrived at the first fight with Itachi and Kisame.