I think Blizzard could give the bard a different kind of mobility: allowing the bard to cast spells while moving as baseline. Either as baseline, or on quick-recharge cooldowns.
One possible idea is just like music is composed by strumming together key notes, the bard could have some sort of 'combo' for their abilities, like you use one ability, and one, two or three other abilities get a special bonus effect: increased damage/healing, healing a second target, healing and putting a shield, damage and a dot, etc.
Another idea is that the bard could 'build' their songspells by 'comboing' notes. Let's say, for example, the bard has four notes: A, B, C and D. After selecting, say, three notes, a spell is cast, depending on the notes. For example: AAA is damage, AAB is damage AoE, ABA is healing, ABB is healing AoE, ACA is personal defensive, ACB is group CD, etc. It's how the old Magicka game works, for those who know it. Of course, this particular idea is much trickier to implement than the others.
Also, another possible difference is that the spellcaster DPS spec could be based on Holy damage, which would differentiate it from the other caster DPS.
I think the bard could easily have three specs, and even four specs: healer, spellcaster, melee damage is one possibility; healer, spellcaster and ranged damage is another. And, if Blizzard is feeling particularly generous: healer, spellcaster, melee and ranged damage. Four specs.As to the implementation: Healing / supporting seems a nobrainer, but i have too little faith in breaking up the triune of roles to really consider a pure support specc, as such they'd need a dps specc, or weirder: a tank specc.
My opinion is that the melee spec would be a combination of melee attacks combined with sound magic, mostly instant-casting, or building stacks to make a spell instant cast.DPS could be an interesting combination of mental/shadow magic and audio/shockwave/wind magic, maybe borrowing a bit into themes of decay as a nod to metal music.
Myself, I don't see the bard having a tanking spec. Feels too shoe-horned in, IMO.Tanking seems most difficult to imagine, it'd have to be a dancer/dodger sort of thing i'd say, using magic to steel yourself as well. Seems unlikely though.