If you want to buff the spell then I'd go with something other than raw damage, since obviously you don't want it outshining things like smite or sneak attack too much. My first thought would be to make it weaken enemy AC, either lowering the value per hit or by imposing advantage to attackers. In this way it could function as a higher-risk damaging version of faerie fire. Another option is to give it moderate range, letting the druid swing it to send out waves of flame in a line attack. Alternately, you could have it function defensively, giving a buff to cold resistance, increasing AC, or shutting down opportunity attacks from enemies. Lastly, you could have swings ignite enemies for a DoT effect, which could increase with subsequent hits. The low damage would be mitigated unless the enemy wastes time extinguishing itself, and the flame blade doesn't overshadow other class features.