You could include characters from other crime - themed games like Tommy Vercetti from Grand Theft Auto: Vice City. His experience in the crime world could create some interesting dynamics with the Mafia 3 characters. They could either be rivals or form an uneasy alliance.
John Marston from Red Dead Redemption could also be a great choice. His cowboy - like honor and skills in a different kind of lawless world could be a unique element in the fanfic. He might be brought into the Mafia 3 world for some reason and have to deal with the urban criminal environment.
Sherlock Holmes could be an interesting addition. His deductive skills would be a contrast to the more brute - force and street - smart ways of the Mafia 3 characters. He could be investigating a case that involves the Mafia 3 underworld, and the interactions between him and the criminals would be full of tension and mystery.
In a crossover with 'Powerpuff Girls', the Professor could be an interesting character to include. He might study Zim's technology and try to find a way to use it for good, while the Powerpuff Girls are busy fighting Zim's minions. And Zim would be both fascinated and enraged by the super - powered girls.
Definitely Deadpool. His chaotic and fourth - wall - breaking nature would add a whole new level of craziness to the Suicide Squad. He could have some hilarious interactions with Harley Quinn, like competing for the 'craziest character' title. And his regenerative abilities would make him a valuable asset on dangerous missions.
From other franchises, Link from The Legend of Zelda could be an interesting addition. His sword - fighting skills and puzzle - solving abilities would contrast with Sonic's speed - based combat. He could be on a quest in Sonic's world and end up teaming up with Sonic to defeat a common enemy. Another character could be Samus Aran from Metroid. Her powerful weapons and armored suit could bring a new level of badassery to the fanfic, and she could have some interesting interactions with Sonic and his friends.
In a crossover with Sailor Moon, Sailor Moon herself would be a great addition. She has her own powers and a strong sense of justice, just like Ladybug. Tuxedo Mask could also be interesting to pair with Cat Noir. Another good character could be Naruto from the Naruto universe. His ninja skills and determination would blend well with the Miraculous Ladybug world.
If it's a crossover with 'Doctor Who', the Doctor would be an amazing addition. His vast knowledge of different cultures and species could help the BAU in cases that have an otherworldly aspect. And his time - traveling ability could add an interesting twist to the story.
If it's a crossover with Metroid, Samus could be a great addition. She has her powerful suit and weapons, and her serious, lone - wolf attitude would contrast with the more innocent and determined protagonist of Cave Story. They could team up to fight a common enemy that is threatening both their worlds, like a race of bio - mechanical creatures.
In a crossover with Danny Phantom, Danny would be a key character. His ghost - fighting abilities and the Totally Spies' skills in dealing with more earthly threats could combine in an interesting way. For example, they could face a villain who has the ability to phase between the ghost zone and the human world, and Danny and the spies would have to use their respective skills to stop him.
Well, Hermione Granger would be a great character to put in an 'alanna the lioness crossover fanfic'. Hermione is extremely intelligent and well - versed in magic, and she could exchange knowledge with Alanna. Then there's Buffy the Vampire Slayer. Alanna, with her combat skills, and Buffy, with her vampire - slaying expertise, could team up to fight supernatural threats that are common to both their worlds. And don't forget about Legolas from 'The Lord of the Rings'. His archery skills and elven magic could complement Alanna's own abilities in a crossover story.
In a crossover with 'Fairy Tail', characters like Natsu would be great. Natsu's fiery personality and powerful magic could contrast well with the Shield Hero's more defensive and strategic approach. They could have some epic battles together against a common foe, like a powerful dark wizard who is causing chaos in both their worlds.
Ahsoka Tano would be an excellent addition. She has a rich history in the Star Wars universe, from the Clone Wars to her later appearances. Her wisdom and combat skills could be a great asset to the Rebels in a crossover. She could also provide a link between different eras of the Star Wars story.