Building integrated tercios with pike and shot would be great but the arquebusiers would need to behave very differently than the pike. That makes the unit AI much more complex. When you would order a such a tercio unit to attack, what should it do? Advance to firing range, halt, and volley away while the pikes assume a defensive stance? Or march into melee and force the shot to file to the rear?

Having different actions happening within the same unit at the same time sounds problematic.