There are a lot of manufacturers in this era however two nations that are underdone are the Spanish and Ottomans. One advantage of an Ottoman range is they would work for most of the 18th century as well.
Another less supported, in plastic at least, would be Portugal as they were involved throughout the campaigns in Spain.
In relation to the major nations some plastic gaps are Russian Cossacks and Prussian Landwehr cavalry. Artillery is very expensive in metal or resin so that could also be an area to look at.