To maximize utility:
The proper hull bits to allow the Kt-130 and Kt-133/134 flamethrowers to be built correctly would be ideal. A person could then get as many kits as desired to make any of the 8 different varieties. Would make this kit a killer deal! No "later, when they get around to it, resin pack" needed.
The Finns captured a LOT of these, and outfitted them differently (weapons). So perhaps the half-sprue could be used to support captured Finnish variants?
Those would at least be "fighting" vehicles. Nothing against artillery tractors, but I would think most gamers are looking for firepower, and so anything that adds to this kit to allow more shooty options, the more kits they will sell.