Self Insert Replacement fics are pretty standard affairs. You take the basic idea of an SI, that is to insert the author into the fandom, but twist it so that instead of being their own person, they replace on of the characters in that fandom, this being either a 'soul-swap' (I.e. you inhabit their body) or the less popluar 'body-swap' (Replacing them completely - body and soul - but still being reacted to the same way). Almost always the replacer has the memories of the person they replaced on top of their own.
This chalenge asks you to do a SI Soul-Swap Replacment fic.
But here's the twist.
Most SIRs replace the main character, or a fan favourite. Evangelion is almost always Rei or Shinji; Harry Potter is usually the titular character, or one of the girls; Naruto usually has Sasuke or Sakura being replaced. However, have you ever seen Asuka being replaced by an SI? Ron Weasley? The Kyuubi no Kitsune?
No. So that's what I want you to do. It doesn't have to be these characters, but pick your favourite fandom, then ask yourself "If this really happened to me, who would I least like to be forced to replace?" then replace them.
Hope you have fun exploring your fandoms from a different angle.