Perhaps a powerful wizard in a fantasy setting. If Hope has some sort of special power that could disrupt the wizard's plans for domination. The wizard sees her as a threat and tries to stop her by sending various magical creatures and obstacles her way.
In a more modern story, it could be a corrupt businessperson. If Hope is involved in a community project that is standing in the way of this person's profit - making plans. The businessperson will use all kinds of underhanded tactics like bribing local officials or spreading false information about the project to try and stop Hope.
It could be an envious classmate. Let's say Hope is really good at academics and is always getting top grades. This classmate, out of jealousy, starts spreading rumors about Hope, trying to isolate her from her friends and undermine her confidence.
A powerful sorcerer who has been consumed by despair could be the main antagonist. This sorcerer might see the mage of hope as a threat to his plan of spreading darkness and hopelessness across the world.
The main antagonist could also be a new and unknown force from the Underworld. It could be a powerful spirit or entity that is spreading despair among the demigods. Percy tries to fight against it but finds it so difficult that he begins to lose hope in the face of this seemingly invincible foe.
Maybe a powerful witch who has been corrupted by dark magic. She wants to use the power of both Mystic Falls and Hope to open a portal to a dimension full of dark creatures. She's been secretly manipulating events in Mystic Falls for a long time and now that Hope has arrived, she sees an opportunity to carry out her evil plan. She can use her magic to create all sorts of illusions and traps to stop Hope from interfering.
A powerful sorcerer who wants to enslave all the insects for his own dark magic could also be the main villain. Mosqito Girl would have to use her wits and powers to stop him.
It could be an envious rival. Maybe there's another girl who is jealous of Pyro's unique abilities or charm. She constantly tries to undermine Pyro and create obstacles in her path.
A jealous witch could be the main antagonist. She sees the Sunlight Girl's popularity and power, and out of envy, she tries to cast dark spells on the girl to dim her light.
Perhaps a character who is allergic to balloons and wants to ban all balloon - related activities in the town. This would put the balloon girl in direct conflict with them as balloons are her passion.
There could be a new type of gem hybrid that is half - corrupted. It has some of the powers of a corrupted gem but also some of the intelligence of a normal gem. This hybrid could be attracted to the unique energy that Mystery Girl and Pearl give off and wants to either absorb it or use it for its own dark purposes.
Perhaps a jealous rival. This person might be a male who is threatened by Ed's success or popularity. When they discover Ed is a girl, they could use this secret to try and drive her out of the group or community she's in. They might spread rumors or create problems for her.
It could be Marik. He might see Yugi's new form as a sign of weakness and try to take advantage of it. Since Marik is often power - hungry, he could target Yugi more aggressively, thinking she'll be an easy target now that she's a girl.