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I've never seen a scenario where Castile doesn't become 'Spain' of a sort, whether it is with Aragon or Portugal.
So the challenge is with a POD after 1400, prevent Castile's ascendancy in the Iberian Peninsula
Bonus points if Aragon or Portugal become the dominant power
Super bonus points if you create a strong Navarre
Ultimate bonus points if you can extend muslim rule in the south

(I mean Granada-esque rule, not the new Al-Andalus)