The Legacy of the Glorious
The Legacy of the Glorious (which can be read here) is an AH, written by Milarqui, that deals with a Spain that manages to regenerate itself after the many problems that take place during the 19th century. There are some parallels with A Prussian on the Spanish Throne, but it grows on a different path than Tocomocho's timeline.
The timeline itself begins at La Gloriosa Revolución of 1868, which exiled Bourbon Queen Isabel II and her family to France and tried to establish a constitutional monarchy in Spain. In RL, after two years, Italian prince Amedeo di Savoia, the second son of then-Italian King Vittorio Emanuele II, became king of Spain, but resigned two years later, fed up with Spanish politics and instability.
In The Legacy of the Glorious, the man that becomes king is Prussian prince Leopold zu Hohenzollern-Sigmaringen, who becomes chosen because a telegram that, in RL, contained a mistake, does not have it in this TL. This leads to Spain participating in the French-Prussian War (here called the Hohenzollerns' War) and eventually becoming, while not a Great Power, still a power to take into account.