I've just finished building a Rubicon Flak 88 37 and a PAK 40, so I'm in the mood to come here and join this wish-listing area.
If we're talking about useful kits, we have to be talking about kits that of course have a broad appeal across different factions even. So for me, at the top of the list, is:
"Universal" Kits:
The M5 Halftrack:
What makes this one work? Well, to be short, this is the Halftrack that was utilized by almost every allied country of World War II. Canada, France (Free French), Britain, and the Soviet Union all received this Half-Track as an export model from the United States. If the sprus came with British/Soviet crews, you'd cover x3 of the x4 nations. This kit would be great for collectors and for gamers. Right now the only kit on the market at 28mm scale is the Warlord Resin one.
The M3 Scout Car (crew)
If I'm not mistaken this rendition only comes with an American crew. Again, Britain and the USSR got these IIRC, if there were alternative crews, that'd be a big boon.
The Universal Carrier: Cheap little transport, Commonwealth and Soviets used them.
The Germans:
Right now there is a fantastic selection of German units and weapons already. They had such a huge range, that you'll never fully cover them (lol), but if we were to see a few outliers, I'd say the current ones are...
Field Guns: Howitzers. Both the LEFH and SFH would be great additions to the range, both for players and for collectors.
Mortar Teams: We've got the MG teams and AA guns, why not have Mortar crews?
The Red Army:
Mother Russia could use some Love. There are some holes in the lineup, from Rubicon (and everyone else), so if we're talking practical choices?
Field Guns and Anti-Tank Guns: The 7.6cm field gun, the 122mm howitzer, the 152mm howitzer, hell, even the 203mm Stalin's Sledgehammer (though this one is definitely not practical). Field guns in general are desired by gamers, and offer interesting choices for modelers. The 100mm Anti-tank gun, and even the 85mm anti-air gun, would be welcome additions as well. Though if we're talking about what is most likely to sell, ergo be the most practica: The Howitzers are the top of the list.
Heavy Tanks: The KV-1, KV-2, and IS-2 come to the top of the list, without a doubt.
The Steel Wolves: T-70's and T-60's, though I'd not put these at the top of a practical list, as they're not quite as well known, and I'm not sure of the appeal to the broader audience.
The United States:
Broadly well served, and they have the upcoming Pershing, so for the US, they'll fall into a familiar mold:
Field Guns: Howitzers and AT guns. I see the Quad-50 anti-aircraft guns are coming, though apparently these are resin, which makes me very sad, lol.
The United Kingdom:
They have a ton of stuff coming it seems. Do they have the Churchill coming as well? If so this list is almost over. Even their field guns are on the way, with a variant of the 17 pounder and the 25 pounder in plastic. Perhaps the Bofers AA gun, or some other variants, but by and large, unless someone wants to find something more obscure, I think Britain's being pretty well served (barring the M5 and Universal Carriers).
If we're looking at early war, perhaps the Valentine, or Matilda?