Perhaps a group of dark witches could be the main antagonists. They might be the ones who caused the problem in the first place and don't want it to be fixed because it gives them an advantage in the magical world. They could use their dark magic to try and stop Ben, Rose and Melinda at every turn. For example, they could set up magical traps or send dark creatures after them.
There could be a powerful demon as the main antagonist. This demon feeds on the chaos that is being caused due to the situation that needs to be fixed. He might try to manipulate other magical beings into working against Ben, Rose and Melinda. He could also use his own powers to directly attack them, like summoning fireballs or creating illusions to mislead them.
It could be an evil warlock who doesn't want Ben, Rose and Melinda to fix whatever they are trying to fix. Maybe he has his own plans for the chaos that is being caused and sees their efforts as a threat to his power.
The fanfic could be centered around a spell gone wrong that is affecting the normal lives of witches and warlocks. Ben, Rose and Melinda are the ones who are given the task to fix it. They first need to figure out what exactly went wrong with the spell. This could involve talking to different magical beings who were affected by it. As they dig deeper, they find that the spell was tampered with by a group with malicious intentions. They then have to come up with a counter - spell while also protecting themselves from the attacks of this group.
There could be a fanfic where all four characters - Ben, Rose, Melinda, and Bianca - are brought together by a prophecy. They initially don't get along well but as they try to fulfill the prophecy, they realize they need to put their differences aside. This could lead to some really interesting character development and exciting magical adventures.
Love and relationships could be a common theme. Since it involves different characters like Ben, Rose and Melinda, there might be stories exploring their romantic connections or friendships.
A fanfic could center around Ben teaching Melinda a new and powerful spell. He might be an expert in a certain type of magic that she wants to learn. As they practice, they accidentally create a magical mishap that they then have to fix together. This could lead to a deeper understanding between them.
I'm not sure of specific 'charmed ben rose melinda fanfic' off the top of my head. But you can try searching on fanfic platforms like Archive of Our Own. There might be some great stories there written by fans that involve these characters.
I'm not sure specifically as it could be a very specific fan - created story. It might be a 'fix - it' fanfic related to something named 'Charmed Rose Veran', perhaps it aims to correct or improve upon the original plot, characters or events in the source material that 'Charmed Rose Veran' is based on.
It could also be a magical being from another dimension. This being might have caused Chris's amnesia on purpose in order to study the Charmed Ones' reactions. They could be interested in the power dynamics within the family and how they deal with a member who has lost their memories. Their actions could range from simply observing to actively trying to disrupt the family's harmony.
Maybe a malevolent scientist who has a grudge against female heroes. He believes that only men should have such powers and sets out to take the Omnitrix from Ben and use it for his own evil purposes. He underestimates Ben because she is a girl but soon realizes his mistake.
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.
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.