Perhaps one of the rarest sort of models is plastic artillery, I only know of two, that being Warlord's Wars of the Sun King's kit and Perry's Civil War Artillery. The exotic nature is furher compounded by the total lack of any sort of limbered artillery or options therein.
I think if WGA were to make a few artillery guns with optional limbers that could connect up to the prexisting horse sprues, and then sell seperate crew options i.e "Ottoman Artillery Crews" and give some options like the Jannisary hats, turbans, helmeted heads and whatnot, or a "Western European" kit with sallets, morions, hats, and even uncovered heads would be of great use to those intending to build armies before end of the age of pike.
Of the producers of model artillery in general only four make early modern or late medieval artillery, and its all metal with no options, that being Warlord (Pike and Shotte), Perry (Metal War of the Roses and English Civil War), Milton SG, and Old Glory, and none to my knowledge make war wagons period. If I had to choose there'd be a mortar, serpentine, bombard, and something like a leather cannon, I think they could all use the same carriage if I recall what some of the ligher mortars looked like in the period.
Nextly is peasants, not so much civillians but peasants with the newfound technology of gunpowder, either French Prodestants, Dutch Rebels, or members of the Great Peasants War in Germany, there are plenty more rebellions, and most tend to dress similarly once you strip away the color anyways. Peasant rebellions were very common in this period and I think could make a good kit, be they militias, relgious zealots, people fed up with the current state of things, or an imaginary nation's troops. I think its a good idea anyways.
Lastly is war wagons, now I know WGA hasn't done any vehicles, but the horses are there and a wagon is farily easy to design and make being a few sheets, an H shape and four wheels. An optional half cover could be included and with one kit you'd have something fit from the bronze age up until today. War wagons, poineers heading to Oregon, or just a farmer heading to town, plus if you added a war wagon option you could slide a cannon into it like the Hussites under Jan Zizka.
Perhaps my head is in the clouds but I think a few "regionally ambigous" kits expecially in the realm of artillery and wagons would be of incredible help despite me not buying any models until I finish the current 200+ strong pile of shame mostly consistent of stuff from here.