When I remodeled my home, in anticipation of replacing an old gas water heater with a new electric heater, I had a 10/3 cable run to the basement utility room. Now the heater has been delivered I discover it only needs 10/2 (black, red & ground), 30 amp. A side problem is that there's no light or outlet in the room. Could I use the redundant wire to supply lights and an outlet on another breaker? As I see it, it would involve using the ground wire as the neutral for both circuits.
Not in this case, because you can't put lights or outlets on a 30A circuit.
Note that also, you can't have receptacles on a circuit where more than 50% of ampacity is needed for hardwired loads.
So it could happen if, say, the numbers worked out (20A circuit, 240V/7A mini-split, then yeah, you could use both legs for 120V stuff).
Honestly, the 10/3 cable was a blunder. Water heaters don't need 3-wire feeds. You could, I suppose, fit a subpanel there, with a 30A breaker for water heater and a 15/20A outlet for lights -- however, you'd have to do a load calculation for outlets, because a water heater uses most of the feeder ampacity.
But for a subpanel feeder, for the same price as 10/3, you can get 2-2-2-4 Al feeder which is good for 90 amps instead of 30.