It could be the original main characters from the source material, just older and more experienced. For example, if it's a fanfic based on a teen drama, the once - young protagonists are now adults with more responsibilities.
The main characters would likely be Callie and Arizona. They are the core of the 'calzona' concept.
Obviously, Naruto himself would still be a main character, as he is now an important figure in the village. His son Boruto could also be a main character, as he might be following in his father's footsteps. And Sasuke would likely still play a significant role, perhaps as a mentor to the new generation.
Most likely the children of the main characters from the original series. Like James, Albus, and Lily Potter. Also, the kids of Ron and Hermione, like Rose and Hugo.
There could be new business partners for Christian. As his business might have expanded over ten years, new people with different personalities and motives could enter his professional life, and this could spill over into his personal relationship with Ana.
It could be a young orphan. Orphans often struggle to find a place where they belong, so this character would have a strong motivation to search for that special place. They could be kind - hearted but also have a bit of a feisty side to survive in the world of the fanfic.
Jackie Estacado from 'the darkness' is an obvious choice. His connection to the Darkness power makes him a central figure and his skills and personality can drive the story forward in a crossover.
A cunning spy could be a main character. They are always using artful maneuvers to gather intelligence and avoid getting caught. Their skills in deception and stealth would be central to the story.
Perhaps an old wise man who has known about the Bodacious Peach for a long time but has kept its existence a secret. When trouble arises related to the peach, he has to step forward and use his knowledge to protect it or guide others.
I don't know since I haven't read it, but it could be a group of friends who like to play pranks on each other.
Well, Ben is the central figure. But don't forget about Kevin Levin. His ability to absorb and use different forms of energy could be very relevant in the Warhammer universe filled with all kinds of strange energies. In Warhammer, a Chaos Sorcerer could be a main character. The way his dark magic would interact with Ben's powers and the overall plot would be full of potential. Also, Rook Blonko from Ben 10 could be important. His skills and loyalty could add a different dynamic to the story as they explore the Warhammer universe together with Ben.