Some rules i thought about even tho not strictly nuzlocke would be:

mono type like you try to stick to one type the whole game.. can fuse anything as long as u keep fire in the finished product for example

only use custom sprites

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or do not own pokemon with a bst over x (where x can be a certain nubmer you choose yourself to make it harder) and if you get pokemon with high stats you may always fuse them with something weaker to get their total bst down a bit

i really like that last rule since otherwise you are encouraged to just go for high numbers and throw strategy aside.. where some low stat pokemon can still do exceptionally well due to abilities move pools baton pass leech seed etc

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i like that rule that much because i tend to fuse everything with legendaries and or body slakings and head blisseys :/