Spain is one of those civs who have a lot of sensible pathways, so it's going to get quite exciting once we get some modded civs
In the first age it can start from Celts, Iberians, Carthage, Rome, matbe even Goths (Visigoths) if you stretch it
Goths (Visigoths), is not even a stretch. List of Goth kings of Hispania was a recurring meme as some of the pointless things to learn in old Spanish history textbooks, but proves the high regard in which they were considered for Spanish history. Still in modern times, the basis for a “unified” Spain (besides Catholic Kings and so on) is traced back to Leovigild-Recared unification of the peninsula and conversion to Catholic faith (from Arrianism). So, goths are pretty central on Spanish history.
More of a stretch, but still plausible, would be an historic path from an Islamic polity (as they for sure did leave as well a relevant footprint in Spanish culture, even if nowadays is not fashionable to recognize it). Abbasids could not be, though - besides age, Al-Andalus stayed proudly Ummayad when the Abbasids took power in the rest of islamic world.
Age, as I don’t see Spain moving from exploration age to modern, being such a power on that age and strong in its major objectives (exploration, colonization, movement of goods), while for many centuries weaker in modern ages and modern age objectives (industrialization, applied science,...). So unfortunately I think we are not likely to see Castille or Aragon (which, btw, is a minor but has remarkable Uniques that could be nice to see, but is difficilt to fit)
Based on what we have seen so far with Meji Japan and The Mughals being modern civs. Think it’s unlikely we will see many current national states. Ottomans will likely be modern.
Same reason why i think Spain will remain in exploration age, is the same I don’t think Ottoman empire will be Modern either (Existing or not a modern Turkey in modern era). Strengthts and remarkable feats of ottoman empire are expansion and control of the trade in the Silk route, which I feel more suited for exploration Age, while falling behind in Modern age until being the "sick man of Europe".
On the other hand the same argument (in part) validates the efforts of Russian Empire for being Modern (altough this makes the Soviets fall out of place, as they won't make the cut for a post-WW era), with maybe Muscovy (or Kievan Rus, although it might bee too erly and touchy nowadays) as predecessor in Exploration Age.