In my, not-so-humble, opinion alternate history based on a 'what if' like Shannon Selin's book is good in as much as it keeps closely to the known events and characters.

Historical fiction about real characters, ditto when the gaps are filled in with invention consistent with the reality.

Fanfiction where the character is either historical or based on someone else's novels but is given a major makeover to suit the author seems a debased form to me and insulting to the historical characters or to the original author.

Susan