LSCatilina in DeviantArt had made a series of maps and stories depicting a surviving Kingdom of Gothia, set in Septimania, parts of Aragon and Catalonia, speaking a form of Gallo-Romance very similar to Occitan, but called "Goth". Very interesting deviation.
In Spain itself, Valencia and Aragon tried to rebel in 1640 (very unsuccessfully) and had things gone in a different way, in the same years a plot hatched by the Duke of Medina-Sidonia could have sparked an Andalusian nation in Southern Spain, at the same time Portugal restored (as they call it) its independence. I say could because there's debate on whether the Duke wanted to be King of Andalusia, or else he was merely provoking a rebellion in order to force the king to remove his minister Olivares from office after being blamed for the rebellions in Portugal, Catalonia and Aragon (which also saw a similar plan imagined by the Duke of Híjar, although it had almost zero popular support).
A possible future that has always entertained me greatly would be:
What if Byzantium survived, that is, what if the Ottomans failed to grow and become a military elephant? What of its culture, its politics, its allegiances? It probably wouldn't be a very powerful nation during the Concert of Europe, but the Balkans would be a different place. Probably it would have very strong Italian and Eastern influences within, since by 1400 Italian merchants had all but colonised the remains of the Empire.
Also, Burgundy. The Impossible State also was the richest principality in Europe. It seems the Dukes of Burgundy had, along with the most well-planned matrimonial policy until the Habsburgs grew to prominence, a very pernicious habit of going to war all the time. Charles the Bold died, sure, but had Charles had a son, would we now talk about a Burgundian nation? A Burgundian revolution?
Have I said I love playing as Byzantium and Burgundy?