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alternatehistory.com
See the map below. Your challenge is - should you choose to accept it - to think of a backstory, a timeline, points of divergence from actual history etc. so that (1) Aragon never unites with the rest of Spain (whether under Castilian supremacy or another kingdom's), becomes the main power in the Mediterranean, and acquires/keeps possessions in Occitania; and (2) the Italian thalassocracies of Genoa and Venice are 'stronger' up until at least the seventeenth, preferably the eighteenth century.