Well what I see the Currasier as being is
1. available early, ie very close to Muskets
Because the Conquistador seems like it should be available reasonably early, and so as to allow some gap for the Cavalry
2. Should not Easily beat Musketeers (because they are supposed to get extended in usefulness)
3. Should probably be beaten by Grenadiers (which would be available later)
Perhaps Muskets can get a +25% v. Mounted, and the Currasiers a 10+25% v. Gunpowder (or Free Pinch? to prevent them from getting +50% v. Gunpowder), and perhaps a few other bonuses like Flanking v. Trebuchets, and a % Retreat. They would not be so much of an Upgrade to the Knight as an Alternate (one more focused on fast hitting v. Heavy Hitting)
So Knight v. Pike
->Knight v. Musket
->Currasier v. Pike
->Cavalry v. Riflemen
So field Muskets would be uncountered until Currasiers came along
so Pike-Mace-Knight
would become
Pike-Musket-Currasier
Grendiers would still be slightly vulnerable to Currasiers (12 v. 12.5), but Pikes would still be available and cheap. (12 v. 10)
The Conquistador could have the same boosts (+50% v. Melee, uses defensive terrain) making it a still super dominant unit (10 v. 9s for its best counters, Muskets and Pikes...Elephants would still beat it though)
Edited in response to Roland's below
The possibly Balanced, but less Historical would be my earlier Idea of 13str -25% Gunpowder
10.4 v. Musketeers, and able to beat anything else, Grenadiers would be the only counter in that case, but they would probably be too late.