In some fanfictions, there could be original characters created just for the story. These characters might interact with Ben and his 'babies', either as friends, foes, or just bystanders. But overall, Ben and the 'babies' remain the key players in this type of fanfiction.
Obviously, Ben Tennyson is one of the main characters. He is the central figure around whom the story revolves. And the 'babies' are also main characters, though who they are exactly can vary. They could be new alien forms that Ben discovers and takes under his wing, or they could be some sort of younger versions of himself or other characters related to the Ben 10 universe.
Another plotline might involve Ben protecting his 'babies' from an enemy that specifically targets them because of their relation to Ben or their potential power. He'd have to use his experience and the various alien forms at his disposal to keep them safe.
In these fanfictions, both Ben and Juniper are central to the plot. Their different backgrounds, Ben from a more science - fiction based world and Juniper from a magical world, create a contrast. This contrast is often explored, whether it's in how they approach problems, how they communicate, or how they form a bond. They are the two main forces that drive the story forward.
Obviously, the Big Chill babies would be the main characters. They are likely to be the focus of the story.
The main characters are Rook and Ben's babies. Rook, who might be from a different background or with different abilities, is the one who is going to interact with the babies. The babies, on the other hand, could have inherited some of Ben's traits. Their interactions will form the core of the story. For instance, if Ben has superpowers and the babies show some signs of those powers, Rook will have to deal with that while getting to know them.
Another possible antagonist could be an alien scientist who was involved in the creation or discovery of the Necrofriggian babies. This scientist might have his own evil plans for the babies and sees Ben as an obstacle in his way. He could be using his advanced technology to try and capture the babies from Ben.
Obviously, the Big Chill babies would be the main characters. There might also be some other characters from the Ben 10 universe like Gwen or Grandpa Max who could play supporting roles.
It's really hard to say. Ben at his strongest has access to some incredibly powerful alien forms. But Goku at his strongest can literally destroy universes. However, if Ben could transform into an alien with reality - warping powers, he might have a chance. But Goku's speed and combat experience are also very high. So it could go either way depending on the situation.
The main characters are usually the baby versions of Sonic and his friends. So, there's baby Sonic, who is likely to be really cute and still have his signature speed, just in a more clumsy, baby - like way. Then there could be baby Tails, with his two little tails still being a prominent feature.
The main characters are usually Itachi and Sakura, as the 'itasaku' in the name implies. And of course, their babies are central to the story as well.
It really depends on the situation. If it's in a magical environment like Hogwarts, Harry might have an advantage with all the spells he knows. But if it's an open area, Ben with his Omnitrix could transform into an alien like Way Big and overpower Harry with his size and strength.