PervOtaku

Personally, I think I expect most fanfiction to take the characters I'm familiar with and put them in new situations, especially situations that could not be done in the source material (crossovers, sex, etc.). If the character changes drastically, even with fully logical reasons, it's not really still that same character. It's essentially creating a branch universe in which the character(s) are totally different. I'm sure there are a lot of good and well-loved stories that do this, but it's definitely going to seem out of the norm compared to a fanfic where the characters behave as we expect them to.