For bards, im not entirely opposed to the idea of a caster using an insntrument but im opposed strongly to pure support (which players pair with bards though they dont necessarily
have to be pure support) and opposed to someone just using solely music as a weapon
in wow.
I think adding instruments as weapons for all cloth casters would be an easy win, the output would be whatever magic cloth casters can use the input however is that the caster calls magic through music. throw in some musical notes in the ability animations and you have a pretty good compromise. IMO if the magic input could be any magic output then the input should be a generic option, like the idea of runecaster someone suggested ages back, just add more casting animations and those things would be covered (instruments for bards, glyphs for runecasters, wands for harry potter fans etc)
Ill admit you are right about healing and tanking being somewhat contrived but i think its the healthy kind of contrived, pure support is self necessitating which is kinda bad, having the holy trinity on the other hand gives some big specialization differences, some players genuinely like healing (hell i dont mind it some of the time) annd in my experience healing in those self sufficient games feels like garbage.. Its nice having roles in a role playing game but at the end of the day as soon as players have a way to measure performance they will and they will get MUCH better at the game for it. Going against that will never work, once the cat is out of the bag its out for good, i think its best to just keep the dps specifically as something sacred and work utility around it, and offensive support simply create a clusterfuck for the data players use to gauge their performance.