I totally agree that WWII horse-drawn and human-drawn carts and wagons, especially German, are something HaT should pursue further. Field wagons and limbers are a great beginning but only a beginning, and competition should be fairly limited (though I understand Valiant produce a If.8 infantry cart, and I believe there are several makes of field kitchen around).

It is not hard to find illustrations of the machine gun wagons; many of the others are rather obscure though.

Trailers might be a useful area to look into as well....