I want to make a guide for writing a good roleswap, and here’s the points I have so far:

1. Keep the characters recognisable. If you swap too many aspects of a pair of characters, then you’re essentially left with the same characters as before, just with each others’ names. In a good roleswap, the nature of a character’s personality should stay the same, but the nurture part should change.

   • Good example: Edward <-> Alphonse roleswaps are very common in the Fullmetal Alchemist fandom (and the author even came up with it first as a gag). Jeminy3's roleswap explorers how this would affect their canon personalities; for example, being a suit of armour only worsens Ed’s self-loathing.

2. Think about the implications. If the swapped characters come from completely different walks of life, it’s going to have a huge effect on the worldbuilding; how society at large treats them, how they came to be, et cetera.

   • Good example: The premise of the Lunaverse is that Celestia turned evil instead of Luna. As a result, Equestrian society has superstitions against the sun and gold.

Notes:

• This is a proof-of-concept. If you think I could word something better, tell me.
• This isn’t just about MLP fanfiction, so I want “good examples” from a variety of fandoms, along with succinct ways to explain the characters to people out-of-the-know.

I asked about this on The Offprint Café Discord sever and got these responses:

More like don't be afraid to alter canon to comply with the swap.
If you're going to roleswap, have the confidence to see that story through.
sticking close to canon isn't the problem. sticking to canon when the swapping of roles would probably work better and be more narratively sound if you didn't is the problem.

The point of a roleswap is to ask: If this character was given this power instead, how would they use it? What would happen?
and then answering that question
very simple - if you're writing a roleswap, but you're not answering that question, then it's a bad roleswap