So...I know I am extremely late on this one and I deeply apologize! My anime cross over would be Death Parade and Gangsta! Both extremely twisted and violent plots with emotional endings.
With Decim being an arbiter of justice and having to judge people based on their last few minutes of dying, it's a hard core job and a bit relentless! Which is why I feel he and the jiggalo from Gangsta would get a long marvelously!
These two make a pretty great duo! One being a TAG and is def and the other a rich boy turned rotten from his drunken dad beating the snot out of him with a glass bottle with rendered him blind in one eye. A Tag is basically a military man with a different rank on how violent his is and how quickly he can kill someone. This kind of twisted cruelty mixed with Death Billiard/Death Parade would be glorious!
Sorry @RosePark for the EXTREMELY late response and sorry I haven't been able to keep up with your challenges lately. Hope you and the rest of the community enjoy my crossover idea! <3