Could it be possible expand the usefulness of the skill sing for bards?
It would give them a nice touch, and taking them closer to the D&D spirit,
and also making them highly valued in a group.
Basically what I ask is making possible to bards to sing without a song, the main difference between singing a coded song, and using the skill sing without a song, would be as a difference that coded songs that give you specifics benefits (like protection against a specific type of damage or other specific beneficial spell), the benefits of singing without song would be increased as long the bard increases his skill with the skill "sing".
At first it would increase lightly the chance to hit and the damage done, and as the level of the skill increases it will affect (and in a more powerful way as the skill increases, sort of sing/perform skill in NWN1) things like: chance to hit, damage done, saving rolls, AC, skill points and temporary hp.
A bard while singing this way could no cast spells or use wands, after he stops singing the effects last for a limited amount of time, maybe it could be even limited the number of times that a bard sings a day.