I'd suggest checking manga forums or dedicated Bleach fan sites. They often have detailed breakdowns and recommendations on where to pick up after a specific part. Sometimes, the author or official sources might provide cues as well.
Ichigo would probably be devastated. His trust in relationships would be severely shaken. He might become more withdrawn and focus even more on his battles and training, using it as an escape from the pain of the betrayal. He could also become more cynical about love and people in general.
Another idea for the fanfiction could be a time - travel plot. What if Ichigo somehow travels back in time and meets a younger Rangiku? He could influence her growth and development, and in turn, she could have an impact on his view of the past. This would create a complex web of cause and effect, and it would be interesting to see how their actions in the past would shape the future of the Bleach world they know.
There's a fanfic called 'The Bond of Hearts'. It's set in an alternate universe where Ichigo and Orihime are childhood friends. It shows their relationship evolving over the years. It delves deep into their emotions and how they gradually realize their love for each other. The character development in this fanfic is quite engaging.
One great aspect of Bleach Ichigo and Rukia fanfiction is the exploration of their relationship beyond the original story. Writers often delve into their deep bond, showing how they support each other in different situations. It could be in the face of new threats in Soul Society or just in their daily lives. Fanfiction gives a chance to expand on their characters and how they interact, which is really interesting for fans of the series.