A simple solution we came up with was to keep fireball as is, with it's incredible radius, and update the damage of Lightning Bolt to 6d8 instead of 8d6. In addition, we give targets disadvantage on their save if wearing metal armor.
This simple change makes lightning bolt feel more strategic and put it right back in the game for all of my players, albeit situationally. The limited targets in a 5x100 range for a tiny bit more damage, mixed with the metal armor penalty, suddenly became a major benefit given the correct circumstances.
To put this into perspective, a 3rd level fireball does average of 28 damage, up to 49 damage when cast with a 9th level spell slot. The change we made to lightning bolt now does an average of 27 damage, up to 54 damage at 9th level. Nothing earth shattering by any means.
Plus now lightning bolt has it's own unique identity as a 3rd level powerhouse spell -- separate from fireball -- with slightly higher potential damage if the caster is tremendously lucky (96 lightning vs 84 fire damage) -- AGAIN, nothing earth shattering if someone was ever that lucky.