In this other world, Ichigo could be paired with a local mage. This mage could initially be distrustful of Ichigo but as they go through adventures together, they start to develop a deep connection. The mage could teach Ichigo about the world's magic, and Ichigo could teach the mage about his spiritual powers.
Rukia could be a great character to pair with Ichigo. Since they have a strong bond in the original series, in the fanfiction, she could follow him into the other world or be summoned there later. Their combined power and friendship would make for an interesting dynamic as they face new challenges.
Uryu could also be an interesting character to include. He might end up in the other world separately from Ichigo and they could reunite. Their rivalry - turned - friendship could add a layer of complexity to the story, especially when they have to work together against new enemies in the unfamiliar world.
Rukia could be a great character to pair with Ichigo in a different world crossover. She has her own set of powers and a calm, intelligent mind. In a strange new world, she could help Ichigo analyze new situations and come up with strategies. Their bond as comrades from the Soul Society would be interesting to explore in a different setting.
One popular character paired with Ichigo is Naruto. Their different power sets and personalities make for an interesting combination. Naruto's positive and determined attitude could contrast with Ichigo's sometimes brooding nature. Another is Luffy from 'One Piece'. Luffy's carefree and adventurous spirit next to Ichigo's more serious approach to protecting his friends can create some great stories.
It could be a powerful wizard or sorcerer in the new world who sees Ichigo as a threat to his rule. Maybe Ichigo's presence disrupts the balance of power that this antagonist has carefully maintained.
His friends Orihime and Chad would probably be main characters. Orihime could be very motherly towards baby Ichigo, always making sure he's comfortable. Chad would be the strong protector, making sure no harm comes to the baby.
A good crossover character could be Naruto. He and Ichigo share some similarities in their determination and their growth as characters. They could team up in the new dimension, with Naruto's ninja skills and Ichigo's sword - fighting abilities making for an interesting combination against the new threats they face.
One possible plot twist could be that Ichigo discovers he has a special power in the new world that is completely different from his Soul Reaper powers. For example, he might be able to control the elements, which would change the way he approaches challenges.
Ichigo could meet Frodo Baggins. Frodo, being from a very different kind of world, would find Ichigo's situation strange. But they could bond over their shared sense of being out of place in some ways. Frodo could teach Ichigo about the importance of perseverance in the face of great odds, and Ichigo could protect Frodo from any potential threats in this crossover world with his swordsmanship.
Aizen Sousuke would be an interesting crossover character. Since he's so powerful and manipulative in the Bleach universe, seeing him in a different setting with Ichigo could create some really complex and intense storylines. They could be forced to work together against an even greater evil in a crossover universe.
Renji could be a key character. He could be a senior student who at first is a bit tough on Ichigo but later becomes a mentor. Orihime could also be there, and she might be in the same class as Ichigo, providing some emotional support and her own unique skills in the academy setting.
Obviously, Ichigo and Dark Ichigo are the central characters. Ichigo, with his complex personality and powers, is always a key figure. Dark Ichigo, being his inner Hollow, brings a lot of drama and conflict. The OC also becomes popular depending on how well - written they are. If the OC has unique powers or a really interesting backstory, they can gain a following among fans of the fanfiction.