I tend to look for fics that explore what their relationship might have been like before everything went so horribly wrong. A lot of writers latch onto the 'Ikusaba' surname detail, spinning it into pre-canon backstories where Mukuro, still using that name, is a devoted protector for a younger Junko who maybe hasn't fully snapped yet. There's a vulnerability there that gets completely erased by the time of the game.
You see a lot of 'caretaker Mukuro' tropes, but twisted. Like, Mukuro making sure Junko eats and sleeps, not out of sisterly love, but as part of maintaining her ultimate tool. It's a dynamic built on utility, not affection, and the best fics make that painfully clear. The 'Junko experiments on Mukuro' trope is another brutal one—testing physical limits, psychological breaking points—all framed as 'research' for the sake of despair.
Honestly, the most compelling stuff for me isn't the outright abuse, but the quieter, more insidious manipulation. Fics where Junko rewrites their shared history through gaslighting, or where Mukuro's loyalty is portrayed as a religious devotion she can't escape, even when she knows it's killing her. It's less about graphic violence and more about the psychological prison.