Maybe a group of rogue warriors from another dimension. They don't follow the rules of the multiverse and see Beerus and Champa as obstacles in their path of destruction. They would constantly attack and cause problems for the two deities.
It could be an evil version of Whis or Vados. They have the same level of power as Beerus and Champa's attendants but with a malicious intent. They could manipulate events to cause chaos between Beerus and Champa and then try to take over.
A powerful, ancient being from a forgotten universe could be the main antagonist. This being could be jealous of Beerus and Champa's power and status, and decides to challenge them to take over their universes.
It could be Frieza. Frieza might sense Gohan getting stronger under Beerus' training and see him as a threat. He decides to stop Gohan's progress by any means necessary.
As far as I know, there isn't a widely known fanfic solely focused on Beerus and Champa's father. However, the Dragon Ball fan community is vast and creative. Some fans might be interested in exploring the backstory of their father, but it's not a common theme that pops up regularly. You could try checking out some of the bigger fanfic platforms and search for related keywords to see if there are any hidden gems.
It could include the origin story of their father. Maybe how he came to be a powerful being in the Dragon Ball universe. It might also explore his relationships with other deities.
One idea could be a cooking competition between Beerus and Champa. They could each bring their own unique ingredients from their universes and compete to make the most delicious dish. It would be a fun way to show their rivalry in a different light.
It could be someone who wants to disrupt the relationship between Naruto and Beerus. Maybe a powerful dark ninja who sees Naruto's newfound connection with Beerus as a threat to his own plans for power.
Perhaps a group of anti - technology extremists could be the main antagonists in an Ultron fanfic. They see Ultron as the symbol of all that is wrong with technology and are determined to take him down, no matter the cost. They might use their own advanced, but more rudimentary, technology to fight against Ultron. This would create an interesting contrast between Ultron's high - tech abilities and their more basic, yet effective, methods.
A rival medicine cat from another Clan might be the main antagonist. They could see Alderheart as a threat to their own status or the well - being of their Clan. So they might use dark magic or some underhanded tactics to bring about his death.
Vilgax could be a great antagonist. He could hack into the gaming systems to try and defeat Ben in a virtual world where the rules are different. He might use his advanced technology to create digital versions of his minions to attack Ben.
The Organization in the Claymore world could be the main antagonist. They already have a lot of secrets and do some morally ambiguous things. If Naruto shows up and starts changing the status quo, like teaching Claymore warriors to be more independent, the Organization might see him as a threat and try to eliminate him.
It could be a new alien species that Ben has never encountered before. This species could be territorial and see Ben as a threat to their planet where he's stranded. They constantly try to drive him away or capture him, which makes it difficult for Ben to find a way to leave.