There are plenty of plastic kits, so a sprue with more exotic heads, arms and accessory bits might be the way to go. 
Yes, conversion bits would be mighty handy. I'm looking at some Tyrolean figures right now and seeing that the female figures especially will be needing haversacks and cartridge pouches at the very least.
All the weaponry options would be great to have on sprue. There are some bits that can be chopped off figures in a Victrix box, but pistols and other hand weapons are in quite short supply. Carbines and a volley gun would be great. A double-barrelled flintlock shotgun would be superb (even if it's not in the rules yet — I've got a pencil).
There are a few dismounted dragoons out there in metal; but otherwise, dismounted cavalry are quite lacking. A Hussar and Lancer sprue with a few head options (including female!) would be pretty universal.