There needs to be another Protestant faction, so I'd say the Protestant Union or the Dutch Republic, who both played a huge role in the war. Spain doesn't need its own faction; I think they're fine being merged into a general Habsburg faction. One question I have is what period of the war is this going to be set in? If you decide to add France are you going to take into consideration the fact that they switched sides midway through the war?
This mod about the time of the "Thirty Years' War" (1618-1648), and no, Spain is Spain, and the holy Roman Empire is the Holy Roman Empire...
Why?
Because Due to problems with the Germans king carlos had to give the Holy Empire to his brother, and all Spanish possessions to his other brother, so it was divided ... And Spain and the Holy Roman Empire were Allies to 1700
Well, shall I really take my time to explain you the HRE and the dynastic connection between Spain and Austria? Well let me put it in this way: It´s fine like it is atm, because there is no actual need for seperating Spain from the Emperor, particular because the amount of possible factions is limited to 5. That´d be a waste of space to implement some other interesting factions like France or Poland. In those days it was more important both thrones (the spanish and the imperial) were held by the Habsburger than the actual designation of the name of a country or even the nationality. And the HRE and Spain weren´t allied, you can´t imagine the HRE like a modern state which concludes alliances or declares wars, that simply doesn´t work.
And there is another reason to unite Austria and Spain in one faction. In several wars (not only in the 30-years) both houses send troops into battle to fought together or send troops to support each other (for example Spinola during the first period of the war in Palatine).
btw: @KArantukki: Very nice uniforms