Most of the times someone suggests a Spanish civ, all they talk about are conquistadores, missionaries, galleons and so. And while I wouldn’t be against adding something of that in the Imperial Ages (and only there), I think the focus should be somewhere else.
And I mean, this game is about the Middle Ages, and while I understand the original AoE2 expansion put the spotlight on a more Modern setting, for the time being AoE4 has been firmly established as a game with a medieval setting.
So, what about almogávares as unique hit-and-run infantry? What about caballeros villanos or infanzones as cost-efficient unique heavy chalvary? Maybe even include the concept of the milicia concejil as a mechanic. What about the Parias es an economic mechanic?
What I mean is, if they were to add a Spanish/Spaniards civ, I believe it should be about the Middles Ages in the Iberian Peninsula rather than about 16th century stuff. Even if they are better-known for their America conquests, I think the “Reconquista” offers enough stuff to play around when it comes to units, mechanics and so.