Medium tracked vehicles with a grid size similar to the tank, unlocked at the same time as the tank, would be a great addition. They could serve as an alternative mid-game option for construction vehicles.
If you're interested in importing sprites, the APC from Fallout Tactics could be a fantastic inspiration. Its design might work well as a cargo car alternative or even as a modular vehicle with an armor grid size similar to the car. Both concepts would add variety and depth to the game."