Hermione is known for her academic approach to magic, so she might have a more refined and precise control over spells she has studied. Percy, on the other hand, could have a more instinctive magic related to leadership and order. Maybe he can magically organize things or people with ease.
In fanfiction, the idea of Hermione being pregnant with twins at Hogwarts can lead to a lot of interesting character developments. Her friends, like Harry and Ron, would likely have to step up and support her. And the Hogwarts faculty might be in a quandary about how to handle a pregnant student. There could also be a mystery surrounding the father of the twins, or perhaps it's a magically - induced pregnancy due to some dark or light magic experiment gone wrong.
They would likely raise them with a strong sense of honor and courage. Percy would teach them about the power of the sea and how to use it for good. Annabeth would instill in them a love for knowledge and strategy. They would also train their twins in combat skills to protect themselves in the dangerous world of demigods.
One plot idea could be their different personalities within the Gryffindor house. Percy is often seen as more rule - abiding and Hermione as more of a knowledge - seeker. Their twin bond could lead to them helping each other grow. For example, Percy could teach Hermione about the wizarding world's politics while she helps him see the value in more unorthodox ways of learning magic.
It could be as simple as their voices. Maybe one has a slightly deeper or higher pitch than the other. Hermione, being intelligent and detail - oriented, pays close attention to these nuances and is thus able to tell them apart.
They're popular because it's an interesting and unexpected pairing. Fans like to explore new relationships that deviate from the original canon. Hermione and Blaise come from different backgrounds in the HP world, so seeing them as twins creates a whole new dynamic to play with.
One common theme is friendship. Hermione is often portrayed as initially being exasperated by the twins' pranks but then growing to really value their friendship. Another theme is adventure. Since the twins are always up for some mischief - making, they often drag Hermione into all sorts of exciting and sometimes dangerous adventures.
A common theme in such fanfictions could be Hermione's struggle to balance her studies and her pregnancy. She's always been a studious character, so having twins while at Hogwarts would really test her ability to keep up with her classes and her magical research.
One great Hermione and Weasley twins fanfiction is 'The Twins' Prank Partner'. In it, Hermione joins forces with Fred and George in their mischievous plots at Hogwarts. It's full of humor and shows a different side of Hermione. Another one is 'Hermione's Double Trouble'. This fic explores her relationships with the twins in a more complex way, delving into their individual personalities and how they interact with Hermione.
One great Hermione and the Weasley twins fanfiction is 'Pranks and Potions'. It has Hermione getting involved in the twins' mischievous plans in a really fun way. The story is full of their trademark humor and Hermione's intelligence shines through as she tries to keep them out of too much trouble while also joining in on the fun.