Naruto himself is often very popular as he is the main character of the original Naruto series. Fans like to see how he would interact with Saiyan elements. Goku from the Saiyan saga is also a popular character to include. His powerful Saiyan abilities make for interesting crossovers with Naruto's world.
One interesting concept could be the fusion of Naruto's ninja abilities with Saiyan powers. For example, Naruto might use his chakra in new ways inspired by Saiyan energy control. Another concept could be the interaction between Naruto's friends and Saiyan characters. They might learn from each other's fighting styles and cultures.
Sure, the Vinland Saga manga is complete. It took some time to wrap up, but now readers have the full story at their disposal. Different people have different opinions on how well the ending tied everything together, but it's definitely worth checking out.
The Vinland Saga manga is excellent. It combines historical elements with intense action and emotional depth. The world-building and character arcs are very engaging.