For writing compelling characters in a Fullbring Ichigo crossover, it's important to give them flaws. Ichigo has his temper and tendency to rush into things. For the crossover characters, give them believable weaknesses. Also, create relationships between the characters. Ichigo could form a mentor - mentee relationship with a more experienced character from another universe. Or he could have a friendly rivalry with someone who has similar powers but a different ideology. These relationships will make the characters more engaging and drive the story forward.
Maybe in a Fullbring Ichigo crossover fanfiction, Ichigo's Fullbring could somehow get mixed up with a magical artifact from another fictional realm. This causes him to gain new and unpredictable powers. He then has to journey through different dimensions to find a way to control these powers while also being chased by powerful enemies who want the artifact for themselves.
Fullbring might make Naruto more empathetic. If his fullbring is connected to someone else's emotions or memories (which is possible in some interpretations of fullbring), he would gain a deeper understanding of others. This could change his relationships with his friends and enemies alike. For instance, he might be more forgiving towards his foes if he can sense the pain that led them to their actions through his fullbring.
There might be a fullbring Naruto fanfiction where the concept of fullbring is incorporated into the Ninja world in a unique way. For example, fullbring abilities could be related to the different chakra natures. So Naruto, with his wind chakra, could have a wind - based fullbring that allows him to manipulate air currents in ways he couldn't before. This could lead to some really cool battle scenes and new strategies.
One interesting Naruto fullbring fanfiction could be one where Naruto discovers a new form of fullbring that combines his chakra powers with it in a unique way. For example, he might be able to fullbring out the essence of the Nine - Tails in a more controlled and powerful form.
Naruto himself is an obvious choice. His ability to adapt and his large amount of chakra could lead to some really cool fullbring combinations. He could bring out new and powerful forms that we've never seen before.
One possible plotline could be Naruto discovers he has a hidden Fullbring ability. He then has to learn to control it while still dealing with the threats in his own world. Maybe he uses it to fight stronger enemies like Madara in a new and unexpected way.
I'm not sure about a specific 'naruto has a fullbring' fanfiction off the top of my head. You could try searching on fanfiction websites like FanFiction.net or Archive of Our Own. They have a large collection of Naruto fanfictions and you might be able to find one with this concept there.
Rebirth City villains plundering the protagonist's luck was a type of novel. The storyline usually involved the protagonist becoming a villain after rebirth and plundering the protagonist's luck to counterattack. You can find some relevant novels to recommend, such as " The Strongest Villain, Plunder the Protagonist's Luck " and " Reborn Villain Plunder the Protagonist's Luck ". These novels described how the protagonist used his special identity and ability to plunder the protagonist's luck and achieve a counterattack. However, the detailed plot and story clues about the villains plundering the protagonist's luck in the city of rebirth needed to be read in these novels to obtain more information.