From an entirely speculative viewpoint, you could argue that GNB's kit is already utlilising other elemental cartridges.

Bow Shock and Lightning Shot are clearly using lightning aether. (blasting zone maybe too but thats just going off of what it looks like nothing solid)

Aurora is very clearly water based.

Heart of Stone could be tenously linked to be using earthen aether to make the barrier.

The only unaccounted obvious ones are Ice and Wind (although you could make a VERY tenuous relation that rough divide and maybe sonic break for wind but that is just pure conjecture)

We know that these abilities must come from the Gunblade itself as thancred can use them despite not being able to manipulate Aether, implying they must come from either cartridges or an innate property of the gunblade itself. The only hole in this is that thancred can use them without someone present in the party to make the cartridge for him, although that could be explained by him carrying around that ammo and not the fire aether ammo as he clearly has cartridges on him even when the other person isn't present in the party from reading his dialogue.