Honestly though the worst fanfic ideas are the ones that are amazing but only work for unpopular fandoms. Because that means only a handful of people will read or care about your fic. (Worse still: It's a rarepair for the fandom)
I think I just got banned from omegle for using 'hard on' in a sentence (As in, "I worked hard on this", not... the other thing) and I have never been more disappointed in both the English language and the omegle moderators.
On a scale of 1 to 10, assuming you like The Disastrous Life of Saiki K AND Fire Emblem Fates, how likely would you be to play a hack of Fates that has Saiki K characters? (If you only like one or neither of these things please ignore this)