When I was doing the Catalan titles, I ran into a wee problem: viscounts.
Historically, what the game calls "barons" were intitled viscounts in most of the Frankish sphere (in France and Catalonia, 9-10th Century counties always had a viscount, sometimes two) - "Baron" is a generic term, meaning "landed gentleman of some importance". You'd have barons, and one of these barons would be X, viscount of Y. Using Baron as a base title in France and Catalonia goes agains immersion.
Meanwhile, in Castile and the other Iberian kingdoms, low-ranking lordships were merely lordships, señoríos (there also more complex forms of weird communal nobility titles, like solares, but let's just brush that aside). Counts existed, as well as marquises (but most of them are of later creation). So the name "barón" doesn't really work there either.
Same goes with Dukes. A Duke was a title that was a big deal in France and Germany, but in England and Spain... there were no dukes. Not until the French tradition of investing family members with duchies (early appanages, basically) seeped through, and then the English created the Duchy of Cornwall around the 1340's, while in Spain the first Duke I've been able to find(Visigothic era Duces don't count) is the Duke of Girona (later Prince of Girona), a title for the heir of Aragon. Castile bestowed the Duchy of Medina-Sidonia onto Juan Pérez de Guzmán in the 1440's, much later (if there's any other dukes previous to this one, I haven't found them).
The title Prince has been used sparcely and almost never in a hereditary sense, more of a descriptor, as in "the great lord of". El Cid has been described sometimes as Pinceps Valenciae. This isn't really a title, it just meants "He wasn't a count, he wasn't a marquis, he wasn't a king... he was a lord, an important one, and ruled over Valencia independently from others". Same thing when Mir Geribert proclaimed himself Prince of Olerdola, only to spite the Count of Barcelona.
The title Marquis could be an alternative to Duke in Spain, even if this is exactly the best solution. But the thing is that, basically, using Dukes and Barons in Spain kinda makes Spain feel like a fantasy land of casltes and dragons a little bit, not like real Medieval Spain.
Do you think translating the English "baron" into the equivalent Viscount would work? What about Dukes?
