Weiss would probably be very protective at first. She has always had a bit of a mother - hen side to her, and seeing Ruby in a more vulnerable, deaged state would bring that out. She would likely try to keep Ruby out of trouble and make sure she was taking care of herself.
A great plotline might involve the deaged characters interacting with their younger selves' memories. Weiss could be haunted by her family's expectations in a new light, as if seeing them for the first time all over again. This could lead to her making different choices and growing in a whole new direction.
Ruby is often a popular choice. Her innocent and determined nature can be explored in new ways when she's deaged. Fans like to see how she would interact with the world around her with a child - like perspective while still having that spark of heroism.
One idea could be Ruby having to relearn her combat skills while being deaged. It would add an element of struggle and growth as she tries to regain her former abilities. Maybe she forms new friendships with characters she already knew in a different way because of her new, younger perspective.
One character that would be really interesting to pair with deaged Loki is Doctor Strange. Strange could be the one to help Loki figure out how to reverse the deaging process or use his new situation to his advantage. Also, if we consider the Avengers, Natasha Romanoff could be paired with deaged Loki. She could teach him some skills that he might not have learned in his original upbringing, like stealth and espionage. And Steve Rogers could also be a good match. Rogers' sense of honor and justice could have an impact on deaged Loki and maybe even inspire him to be a better person.
At first, Batman would be shocked and a bit confused. But quickly, his detective instincts would kick in. He would view the deaged Joker as a new and unexpected threat. He'd start gathering information about how the deaging occurred, in case it was part of a larger, more sinister plan. Batman would also be very careful not to underestimate the deaged Joker. He might try to isolate the Joker in a secure location to prevent him from causing chaos in Gotham. And he'd be constantly on the lookout for any signs of the Joker's old, devious self emerging from this new, younger form.
Well, a lot of 'deaged Hermione' fanfics have the theme of redemption. It could be that in her older self, she made some mistakes that she now has the chance to correct in her deaged state. And of course, the theme of mystery is there too. How she got deaged and what the consequences will be are often elements of mystery in these stories.
Time - related themes are quite common too. Sometimes the deaging is linked to time - travel, and this can lead to themes of changing the past or understanding the importance of events in a new way. There can also be a theme of growing up again, but in a different way this time. Harry might learn things he missed the first time around while growing up, and this new growth process can be a central theme in many deaged Harry fanfictions.
One popular idea in such fanfiction could be Sirius Black being deaged and having to adapt to the modern wizarding world. He might struggle with new spells and technology while still having his old memories.
In deaged Snape fanfiction, the theme of redemption can be quite prominent. It gives Snape a chance to rewrite some of his past actions or at least be seen in a new light. There's also the theme of identity crisis. He's not the same old Snape, and he has to figure out where he fits now. Additionally, the theme of adaptation is common. He has to adapt to his new physical state, new social situations, and new magical abilities or limitations that come with being deaged.