Without Castile, Aragon would have acted independently, focusing on Mediterranean Conquest rather than Colonialism.
Aragon in Danger
With France and the newly established Spain, Aragon would become a very small and very weak country in comparison. But that doesn't mean that Aragon doesn't have a chance of succeeding as a nation. With Naples under a Personal Union, the crown of Aragon already controls the bottom half of Italy. However, if Aragon sits still, there is a good chance that it will be partitioned between Spain and France.
The Fate of Aragon
In this scenario, Aragon will continue to expand, annexing or subject gating small Italian City-States like Urbino and Pisa into her realm. While not best friends, Naples is a loyal member of the Kingdom of Aragon. Once Naples is integrated permanently, Aragon will have a powerful trade base to suck in income, being able to compete with the trade leagues of Genoa and Venice. Expansion into the late Byzantine Empire may prove fruitful to have a gateway to eastern trade, though the Aragonese would have to deal with the growing Ottoman Empire. Expansion into North Africa possible, however it would prove to be invaluable and would be left alone until later (1700s-1800s). As for Spain and France, Aragon needs to have allied one or else it will face invasion back at home. France, who have previously claimed Naples as it's own, would not be as friendly as Spain may be. The new Iberian Power has never exactly hated Aragon, just thought of them as a rival. Seeing as it has surpassed Aragon in strength, it would seek to protect Aragon from France instead of conquering the land, as the Castilians never seemed to be too interested in the Sea. Aragon would survive into the Modern Day, although it will lose its Italian holdings to rebels and the Papal State.