It can greatly diversify the plot. Maybe Naruto's summoned duel monsters have their own personalities and goals. They could interact with the other characters in the Naruto world in interesting ways. For instance, a monster might befriend some of Naruto's friends and help them in different situations. This would add new sub - plots and character relationships to the overall story.
Well, it could change the power dynamics completely. If Naruto can summon duel monsters, he might have an extra edge in battles. For example, against strong opponents like Pain or Madara. He could use the monsters' unique abilities to turn the tide of the fight, which would make the plot more unpredictable as his enemies would have to deal with not just Naruto but also his summoned monsters.
The ability to summon duel monsters can also introduce new elements of strategy to the plot. Naruto would need to learn how to best utilize his monsters' skills and when to summon them. This could lead to him training in new ways and seeking knowledge about the monsters. It might also create a situation where other characters are either intrigued or threatened by his new power, which could drive the story in different directions.