This would increase the range of a ship, presumably in exactly the same way that fitting two equivalent standard fuel tanks would.
It's not clear from your post whether these would go into optional compartments or the actual fuel tank compartment itself, I suppose it doesn't matter.
The net result would be to free up an optional slot compared to an equivalent loadout with the existing modules, reducing the loadout dilemma but increasing utility for (probably) bubble traders that don't want to fit a scoop & maximise on either cargo, passenger cabins or HRP modules. It would provide more ammo for engineered rail guns too, which I think makes it a balancing issue for that mod.
I'm not familiar with the cost of fuel tanks but presumably this one would be more expensive. Credits are cheap now & your proposed modules would immediately make regular tanks obsolete. I suppose if it were an engineering blueprint that doubled the capacity at some cost (add a power requirement maybe) that could be engaging. Call them Guardian fuel tanks
If you are just looking to be able to travel further on a tank of fuel I think making jumps use less fuel as a special effect would be my preference, and I think we already have that so it could just be buffed & save a bit of dev time.
Could you expand on your reasoning for wanting these tanks?