Well, I think we can add some more troops, maybe cavalry (herreruelos and similar troops...)
In the cavalry we have a "problem". The "Spanish Rider" can be upgrade to "Herreruelo", but that word also refers to the "raytres" or "swartzreitters" that were German troops, so the word to use would be "Coracero" or "Cuirassier" that is used for all the non-german "raytres".
I know that the Spanish Imperial Army had soldiers from all over Europe, but the "reitters" or "Herreruelos" can be independent from the Spanish trooptree (special mercenaries), as they looks different from the other riders (you know, the black armour and so...) but also added to the pt_reinforcement of the Spanish faction. Don't know if I have made myself understand...
For the artillery... we can add two upgrades before the Arquebusier, something like "artillery assistant" and then "gunner" or "official gunner" (in Spanish "Auxiliar de Artillería" -> "Oficial Artillero"). There were different names for them depending on the kind of artillery that they used, but to be more general, I think the names are ok.
So, there would be 3 more troops, one cavalry (Coracero/Cuirassier), and two artillery (Auxiliar de Artillería/Artillery Assistant -> Oficial Artillero/Official Gunner).
Damn, when correcting the grammar, it wanted to change non-german to non-human...