Circe could be a great villain. She has powerful magic and could create all sorts of illusions and obstacles for Wonder Woman and Hawkgirl. She might try to turn them against each other or use their powers for her own dark purposes.
Darkseid would be an extremely formidable foe. His god - like powers and his desire to conquer could bring Wonder Woman and Hawkgirl together in a fight for the very existence of their world. They would have to use all of their skills and courage to stand up against his might, and perhaps even find a way to reach out to some of his minions to turn the tide of the battle.
They could both develop a stronger sense of sisterhood. As they face various challenges together, they might start to see each other as more than just fellow superheroes. They could support each other through personal issues as well. For instance, if Hawkgirl has some unresolved issues from her past life, Wonder Woman could be there to help her work through them and vice versa.
Perhaps they are in a competition within the Justice League to see who can complete a series of challenging missions first. This could lead to some friendly rivalry as well as them learning to respect each other's abilities more. Along the way, they might also have to deal with a villain who tries to take advantage of their competition.
Another plot could be a story where Hawkgirl and Wonder Woman have a misunderstanding at first. Hawkgirl thinks Wonder Woman is being too overbearing in her heroics. But then they realize they are more alike than different when they face a threat that targets female superheroes specifically. They then work together to take down the threat and in the process become good friends.
There's also 'A Different Kind of Teamwork' that delves into how Hawkgirl and Wonder Woman form a unique bond. It shows their different perspectives coming together in a really interesting way. This fanfic explores their friendship and how they support each other during tough times in the DC universe.
Friendship is a very common theme. Since they are both strong female characters in the DC universe, many fanfics explore how they become friends and support each other.
They have some shared storylines and might interact in battles or team-ups. But it depends on the specific comic arcs.
Cheetah could be a great villain. She has a connection to Wonder Woman already, but in a crossover, she could be working with some of Batman's rogues like the Penguin. The Penguin could provide the resources while Cheetah does the dirty work in combat against Batman and Wonder Woman.
Lex Luthor could be a great villain. He could be after some technology in Spiderman's world that he wants to use to enhance his power in his own battles against Superman. At the same time, he might see Wonder Woman as a threat to his plans of world domination. He could use his intelligence to create complex traps and weapons. His corporate power would also allow him to manipulate situations in both the worlds of Spiderman and Wonder Woman.
A powerful alien warlord could be the main villain. This warlord might have his eyes on Earth because of its resources or strategic location. Supergirl and Wonder Woman would have to use their unique powers and teamwork to defeat him. He could have an army of minions and advanced technology that makes him a formidable foe. They would need to figure out his weaknesses and come up with a plan to stop his invasion.
Lex Luthor could be a great villain. He could be jealous of the relationship between Superman and Wonder Woman and create a plan to turn them against each other. He might use his intellect to create weapons that can target their weaknesses.