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The spirit of the catch your first encounter rule in nuzlocke is to force the player to use mons they wouldn't normally use.
This only really ensures the majority of runs play exactly the same where you end up with the regional bird/rat and bug of the gen and not much else.
Without resorting to randomizers, what are some modifications to this rule that would keep the spirit alive? In ORAS you can simply only attempt to capture hidden mons in the grass which adds an additional chance of you fucking up and scaring them off.
But in older gens what can you do? Change it so you capture the first unique per zone? That is really only an early game bandaid for when the first few routes are covered in birds and rats.
There is always just going out of your way not to use mons you know you've used in the past, a shaky ground but really you only have yourself to lie to when making those choices.