What is the ruling on a woman imagining herself as a male character and living in a fictional story of her own making, despite living her normal life unaffected?

There is no accountability for mere imaginations and thoughts, for a person is not held accountable for inner whisperings unless acted upon or spoken. This is based on the saying of the Messenger of Allah (peace and blessings be be upon him): "Indeed, Allah has forgiven my Ummah for whatever they whisper to themselves, as long as they do not act upon it or speak of it." However, one should not dwell on these imaginations, and instead occupy free time with what is beneficial.