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.
Another plot could be that Chris's amnesia makes him believe he is a normal human. He gets involved in normal human activities like getting a regular job and making non - magical friends. Meanwhile, the Charmed Ones are trying to figure out how to break the amnesia spell without scaring him off. This would create a contrast between his new 'normal' life and the magical world he actually belongs to.
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.
Most likely, Chris would be one of the main characters. Given his significance in the 'Charmed' universe, he would play a key role. Also, the main archer from 'Archer' would probably be a main character as well, since it's a 'crossed' fanfic.
Based on the title, it seems that Chris and his cousins would be the main characters. But without more information, it's hard to say exactly which cousins are involved or if there are other important supporting characters.
A new vampire who is jealous of Edward's relationship with Bella could be the antagonist. With Edward having amnesia, this new vampire could try to take advantage of the situation and win Bella over. They could also try to turn Edward into a more 'evil' version of himself while he is vulnerable.
The main characters seem to be Caine, Archer, and Chris. Caine might be a powerful magical being with his own set of abilities and motives. Archer could be someone with skills that are either complementary or in contrast to Caine's. And Chris, who is likely to be in a position where he is affected by the actions and relationships of Caine and Archer.
Well, she could be a bridge between two different magical cultures that Bianca and Chris are associated with. Her upbringing could be a blend of both, which gives her a unique perspective. This could lead to her being involved in diplomatic efforts within the magical world to keep the peace.
There could be many interesting plotlines in a fanfic about this. For example, the daughter might inherit unique powers from both Bianca and Chris. Maybe she has to navigate the complex world of magic while also dealing with the family dynamics of being related to such powerful characters.
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.
One common theme is family. Since he is a baby in the Charmed world, his family has to protect and raise him. This often leads to stories about the strong family bonds within the Charmed family.