Hi Simon, this would be a good progression for TtS! for me and I have been giving it a bit of thought. The first thing is arquebus units, in the rules I use at the mo they can be Lt Inf or standard units so that's easy, 3 shots for the first and 6 for the second. Most rules needed are covered by handguns but I think range should be 3 boxes and because of its power it should render all armour benefits null and void. This means unless its save is due to veteran status a minimum 7+ save applies. I'm not sure how realistic it would be but I'm also experimenting with a salvo rule, if not disordered or in rough going standard units can fire 2 shots but end unloaded, this needs a separate activation to remove before further firing. Also for the Spanish swapping pike and shot shouldn't be a difficult activation.
Doppelsoldner - the double pay nutters who charged in to disorder enemy pike blocks. I've been thinking of Lt Inf who hit on a 6+ vs other infantry to give them a bit more oomph.
Reiters - pistol cavalry performing the caracole firing maneuvre then charging in when enough damage has been done. Range 1 for pistols, 3 shots, standard cavalry. When activated too shoot if they hit and damage an enemy they can automatically charge after as well. If 2 units are in the box a doubly difficult activation allows the first unit to fire, swap places with the second who can then fire in turn and charge after as well. Probably more Wars of Religion and later than Italian Wars.
Just ideas, but if you ever want me to playtest anything just let me know I can do Italian Wars no problem.