I agree on some mortars/siege guns. Although it would be nice to have smaller mortars as well for battlefield. Period limbers as well.

Horse grenadiers would be nice. I've made some from conversions but an actual set would be good. And a period set of hussars.

The one set I would like to see in 1/72, which would be peripheral to WSS but fit more GNW would be Polish Pancerni. I've bought various cossack/Red Box sets and plan on headswaps and conversions plus may buy some metal but an actual set of Pancerni with mail armour and pistols/carbines would be popular for GNW and earlier wars.

Other than that, you have done a wonderful job producing the WSS range.