Well, a theme could be the struggle between good and evil within him. In a fanfic, Batim the Butcher might be constantly fighting his own dark urges, which creates angst. Also, a sense of abandonment could be a theme. Maybe he was left alone in his dark world, and this makes him full of angst. There could also be the theme of powerlessness, where he is unable to change his fate or his nature, leading to a lot of angst.
It could be another powerful and malevolent character from the same universe. Maybe someone who wants to take over Batim the Butcher's territory or power, causing conflict and angst for the Butcher.
One idea could be that the Butcher Gang members get lost in an alternate dimension within the studio and have to find their way back while facing strange new creatures. Maybe they form an unlikely alliance with a minor character from the game to survive.
Well, obviously the Butcher Gang themselves are the main characters. They consist of Charley, Barley, and Edgar. Each of them has their own distinct personalities. Charley might be the more impulsive one, Barley could be a bit more cautious, and Edgar might be the most unpredictable.
Some fanfics might put the Butcher Gang in different scenarios, like a modern - day setting outside of the game world. They could be trying to adapt to a normal life while still having their inky quirks, which would make for a really unique and humorous read.
You should also consider the relationships between the Butcher Gang members. Maybe one of them has a secret that they're trying to hide from the others. This can add a lot of drama and intrigue to your fanfic. And don't forget to write in a way that captures the spooky and unique atmosphere of the Bendy universe.
Betrayal is also a prevalent theme. Mammon may be betrayed by those he trusts, or he may feel like he has betrayed others in some way. This can create a deep sense of guilt and angst. In some fanfics, this theme is explored through complex plotlines where Mammon has to deal with the consequences of perceived or real betrayals, and it really tugs at the heartstrings of the readers.
Betrayal is another theme. It could be a perceived betrayal, like when one of them makes a decision that the other feels is a betrayal of their trust. This can lead to a breakdown in their relationship and a whole lot of angst as they struggle to come to terms with what has happened.
One common theme is his loss of divinity or power. When Apollo experiences a decline in his abilities, it often leads to a great deal of angst as he has to come to terms with being less than he once was.
A possible plotline could be Bendy getting hurt while exploring a new and dangerous area in the ink world. Maybe he stumbles upon a hidden trap set by an unknown entity, and this injury slows him down, making him more cautious as he tries to find a way to heal himself.
The exploration of morality is a theme that can be found. In Undertale, morality is a huge part of the gameplay with choices between mercy and killing. In BATIM, there are also moral dilemmas in a sense, like whether to trust certain characters or not. Fanfics might combine these moral aspects, having characters from both face new moral choices in the crossover world. Also, the theme of discovery is there, as characters discover the new rules and beings of the other's universe.