The only non-slash story I have written that dose not bring a fictional character into play is: whip it good.
99.9% of everything I have ever written deals with either a fictional character or false information such as Kevin being Nick and Aaron's father.
Most of us use fictional characters, I don't think that's what they mean. Original characters are used in probably 99% of fanfic (the other 1% being anything written by Mare
).
At one point in fanfic trends most stories featured a BSB as the main character, then slowly original characters started becoming the focal points of stories with the BSB as supporting characters. Like most trends that did start to switch back and I'd say now it's a split between people who use a Backstreeter as the main character and those who don't.
But the new trend seems to be not making the story about the BSB at all. It's told from the perspective of an original character, featuring all original characters, with the only BSB connection being that of a parent.
I believe it can be done well but in order to legitimately say it's fanfic it needs to be about the Backstreet Boys with their children as the supporting characters. The trend seems to be the opposite which,
in some stories, wouldn't come close to being fanfic. You can't write fanfic about someone you made up.