The current fuel tanks are already quite good but there are ways that they could be improved.
Thickness
One setting that could be added is a slider that would change the strength of the tank but would increase its mass and volume. Tanks with very thin walls would be easily destroyed by impact, drag or high acceleration.
Material
Another setting could be the material that the fuel tank is made of. Different materials could have different strengths, masses, heat tolerance and costs. A fuel tank using a material with a high strength would not need to be as thick as one using a weak material.
Heat shielding
This would be a setting that if enabled would add a heat shield around the fuel tank. This heat shield could be made of different materials which would have different weights, heat tolerance and costs.
Suggestion
But then it wouldn't be stock.