United Kingdom of the Spains:
The idea is that the Hispanic kingdoms gradually form into a united kingdom of Spain similar in some ways to OTL Great Britain, just with more steps.
The POD is that Enrique I of Castile manages to avoid his random early death in 1217 preventing Leon and Castile from being united in 1230 under Fernando III. The two kingdoms avoid a early union preserving their separate institutions and customs for much longer.
As the centuries pass, Leon and Castile and the other hispanic kingdoms continue onward with the Reconquista, engage in conflicts and royal marriages among each other, and embark on voyages of exploration until a lucky monarch finds himself inheriting the kingdoms of Portugal, Leon, Castile and Aragon like Charles V of OTL with his inheritance.
Eventually, Granada, which survived as an independent islamic kingdom for much longer due to division among the christian states, is brought into the Spanish crown after a grand invasion which completed the Reconquista - the event that marks the founding of the united kingdom.
Granada remains a separate 'Crown' from Castile to prevent Castile from becoming too powerful at the expense of the other crowns. However, Castile is the dominant kingdom and the castilian language functions as the language of government. The national capital is also the castilian capital of Toledo. Each of the other constituent realms, however, keep many of their old laws, institutions and languages separate from that of Castile and have their own devolved parliaments.
Questions and comments are always appreciated!