Ive often thought a Bard could sort of act like how a Healadin heals you for more the closer they are to you. Bringing Twisting back to life in WoW, where Bards would have an arsenal abilities that correlate to each type of play style. A 20 player raid having up to three comfortably. A bard for the tanks and melee dps, A bard for the healers, and a bard for the ranged dps. Each specializing in a tree that would give them the best songs for the classes theyre playing too. Being able to fine tune to each, or be a jack of all trades for dungeons. Up close getting maximum effect, further away getting extremely low.
It's crazily hard to balance group buffs - see the debacle of disc priest this expansion. Think of even a super simple example - if the bard does no damage at all, and just buffs other people, how much do you buff them by ? Assuming 2 tanks and 4 healers , you have 14 dps spots other than the bard ( 1/2 each for tank ) so they need to buff it by at least 7% to be worth a raid spot over any random dps. This makes them hot garbage in 5 man, but we'll skip that for now.
So if you set it to 7%, if you run 3 tanks and 6 heals the bard is instantly bad, and if you run a 30 man they are grossly overpowered.
But then you have two people who want to be bard, you have one less person to buff and the percentage is off again.
This kind of things leads to over-complicated stupid systems like absolution.
Mixing in tank or heal buffs makes it even worse, because tank buffs in general are either required or useless, and the number of healers required varies wildly even in mythic.