My ideas for the swap roles are don't really follow the "swap" rule, you can imagine the swap rules as Inverted Fate type than Underswap type.

Amy Rose (Rozy Ames) -> Sonic the hedgehog (Lord Mazin)

Miles "Tails" Prower (Airborn) <-> Metal Sonic (Anti "Steel" Mazin)

Dr Robotnik -> The intial bunny concept for sonic.

Sonic the hedgehog (Lord Mazin) -> Dr Robotnik

Rouge the bat (Wings) <-> Knuckles the echidna

Silver the hedgehog / Blaze the cat <-> Kitsunami the fennec / Surge the tenrec

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Might make more of this AU soon.

(trying out more types of content apart from undertale)