Ra's al Ghul is another common foe. His immortality and vast resources can create complex and engaging death battle situations against Batman in fan - made stories.
Superman can also be an opponent in some 'death battle batman fanfiction'. It's an interesting match - up as it pits Batman's brains and preparation against Superman's superhuman strength. Writers often explore how Batman could potentially defeat someone so much more powerful.
One interesting plotline could be Batman facing off against an unexpected and powerful villain in a death battle. Maybe a villain from another universe who has unique powers that Batman has to strategize against.
Common themes often include the idea of justice. With the Joker dead, does Batman's concept of justice change? Is it more about vengeance now? Also, the theme of legacy is present. How will the Joker be remembered in Gotham? And how will Batman's actions be seen in the context of the Joker's death? Additionally, there's often the exploration of the psychological impact on Batman, such as guilt or a sense of emptiness.
Venom is a very common opponent. Given their history in the official comics, it's natural for fanfiction to explore their battles further, especially in a death - battle scenario where the stakes are even higher. Their symbiotic relationship adds an extra layer of complexity to the fight.
The significance can be the exploration of different character dynamics. In fan fiction about the Joker's death in the Batman universe, it allows fans to imagine how Batman would be affected. It could show a new side of Batman, perhaps a more conflicted or less morally certain version as he deals with the death of his archenemy.
Love and attraction are often themes as well. Their opposing natures create a sort of magnetic pull. In many fan fictions, there's a slow - burning romantic tension between them. Also, the theme of Gotham's future is present. Whether Ivy's ideas for a greener Gotham or Batman's way of protecting the city will shape the future of the city is explored in different stories.
A common theme is the contrast between their powers. Batman is a human with no superpowers, relying on his intellect and gadgets, while Supergirl has superhuman strength, flight, etc. Their partnership is also a big theme, like how they learn to work together despite their differences. Another theme can be the protection of their respective cities, Gotham for Batman and Metropolis (where Supergirl also has a connection) from threats.
I don't think it's a common concept. Most Batman fans love the character for his moral code which includes not killing. Fanfiction usually tries to expand on the existing lore in a way that is somewhat in line with the general perception of the character. So, stories where Batman kills are more of an exception rather than the norm in the fan community.
The main characters are Batman and the Joker. Also, Batman's allies such as Robin, Batgirl, Nightwing are important as they are the targets of the Joker's attacks.
One common theme is the exploration of Batman's secret identity and how it affects his relationships. Another could be the power dynamics between him and his partner in the story.