Personally I prefer the emperor tier to be limited, the emperor is supposed to be the universal ruler and secular head of Christendom.
Instead I like to have the title of emperor of Spain and other possible alternative imperial titles, handled like the emperor trait in Ck 1; ideally area specific, so that the king of Castille, León, Aragon, Portugal, Navarra and Galica can get the trait emperor of Spain (the titular kingdoms aren't needed, but they make the event more likely to happen), which gives extra prestige. However this does mean, that these titles will be more regionally, so gaining a emperor trait from random titles all over the map will be harder, if included (IMHO not necessarily needed) it will require more titles.