Make oxide asteroids contain sulfur and carbonic asteroids contain calcite. Right now coal/explosives are made from only one asteroid type, maybe it would be more interesting if they required mixing of two. Also, I find carbonic asteroids to be quite useless on inner system routes. Making them the source of calcite for thruster fuels and foundries might make them more even with metal and oxide asteroids, which both have very critical roles.

That's an interesting idea. However it does seem like a sideways step (more likely to be seen as pointless) as opposed to an upwards step.