Lex Luthor would be a great choice. He could be plotting against both Batman and Supergirl. He might try to use his intellect to create a weapon that can nullify Supergirl's powers and outwit Batman's strategies at the same time.
Darkseid could be a formidable villain in such a fanfic. He sees Supergirl as a potential threat and Batman as an annoyance. His minions could start causing chaos in both Gotham and Metropolis, which would require Batman and Supergirl to put aside their differences and face this powerful foe together.
The main villain could be Darkseid. Imagine if he somehow finds a way to enter the Naruto universe. He would be a formidable foe for both Naruto and Supergirl. His omega beams and immense strength would be a huge challenge for them to overcome.
Lex Luthor would be a great choice. He could be causing trouble in Gotham as well as Metropolis. His intelligence and resources would pose a significant threat to both Supergirl and Batman. He might be trying to manipulate events to make the two heroes fight each other so he can carry out his evil plans.
One common plotline is a team - up where Batman and Supergirl join forces to take on a powerful villain like Darkseid. They combine their different skills, Batman's strategic mind and Supergirl's super - strength and flight.
A really cool story could be set in an alternate universe where Supergirl and Batman are the last two heroes left. They have to rely on each other completely and in the process, they discover new aspects of their powers and themselves. Their relationship could evolve from just being allies to something more complex and deep, with Supergirl teaching Batman to have more faith and Batman teaching Supergirl more about strategy.
Supergirl is also a great character in fanfic. Writers can play with her powers, her fish - out - of - water experience on Earth, and her relationship with Batman. Her naivete about some Earth things combined with her powerful abilities make for interesting stories.
Kingpin would be a very suitable main villain. He has his criminal empire in Hell's Kitchen already, and if he decides to make a move on Gotham, it would bring him into conflict with Batman. His power, wealth, and ruthlessness would make him a formidable foe for the two heroes to face together.
The Joker would be a great choice. He could use some new - fangled chemical or device to mind control Batman. Imagine the chaos he could create with a mind - controlled Dark Knight at his beck and call. He might make Batman do really out - of - character things like laughing hysterically while committing crimes.
Ink could be a great main villain. He's a character from the Batman Beyond series known for his ability to manipulate ink - like substances. He could target Robin specifically, seeing him as a weak link in the Batman - Robin duo. His powers would pose a unique challenge for Robin to overcome.
Another option could be a new villain created specifically for the fanfic. This villain could be someone who has a vendetta against the Wayne family because of something Batman's dad did in the past. He could be a master of disguise and manipulation, making it difficult for Batman and his dad to figure out who he really is.
Supergirl's exuberance and her willingness to trust can sometimes clash with Batman's more cynical view. However, in the context of a romance in fanfiction, this can be a source of growth for both of them. Batman might start to trust more because of Supergirl, and Supergirl might become more cautious under Batman's influence. Their personalities also play a role in how they show affection. Batman might be more reserved in his displays, while Supergirl could be more open and affectionate, which could lead to some sweet and sometimes awkward moments as they navigate their relationship.