If they just threw a 1% damage aura on keeping up songs I would be a happy pickle, they can always adjust personal damage potencies if that ends up making their numbers to high. Just having the illusion of providing a group buff would he enough for me. Why damage instead of crit? Merely for balancing purposes, since crit tends to spiral out of control scaling wise.
I was talking with a FC member on discord and we both played around with and discussed the idea of BRD and DNC having different focuses for buffing party damage, essentially, DNC would have stronger buffs but only buff one person (the dance partner), BRD would have weaker buffs but they would effect the group. The idea being that if you wanted to buff the crap outta one person, you bring a dancer, if you wanted to buff the group at once, you bring the bard.
Balancing issues aside it would be interesting. But I love bard as it's designed, I just want a drop of its support back, even if it's so minuscule it might as well not exist.