Perhaps the fanfic is about Buu waking up and being confused by the events of the Cell Saga. He might not understand the conflict between the androids and the Z - fighters. So, he could start interacting with different characters in his own strange way, like befriending some of the minor characters or causing unexpected mayhem just because he doesn't know any better.
The ending of Vinland Saga manga is quite complex and bittersweet. It involves major character developments and resolutions that leave a lasting impact.
Well, the Vinland Saga manga ends with a sense of closure for some characters while leaving the future open-ended for others. It's a complex and thought-provoking conclusion that ties up many loose ends.