In a way, because it is such a hard class to peg, I could see it being a background.
I could see it being a casting method (performance powers) and this could apply to any casting method like power points (innate powers) or at-will (granted powers) or vancian (study - or default) in this way it could be applied to any caster class and could provide the same flavor of the bard's magic and song.
Also the bard could be a prestige thing. Start as another class then they join a college, like the harpers or something. They had said they planned on making PrCs feat chains. If this is still the case I could see this.
Because it is hard to peg, it makes sense to do this approach, pretty much everything but a class.
I reach the opposite conclusion... It is hard to peg because it is many things at once, thus it can't be a background, a casting method or a feat... those are way way too narrow! Unless you mean, that a Bard should be a background PLUS a casting methods PLUS a few feats PLUS a unique spell list PLUS some special features, all together to "build" a Bard, and that sounds like a class to me
A subclass cannot work because it's still quite small (although larger than each of those before), but more importantly because it is a subclass
of a specific class. So even assuming that 5-6 subclass features could be enough to wholly represent a Bard, do we make it a Fighter subclass, a Wizard subclass, a Rogue subclass? If the original problem is that a Bard is a bit of Fighter, Wizard, Rogue, Druid maybe also Cleric and Ranger, choosing any class as a basis is going to be like pulling the sheet to one side at the expense of the other.
Instead, exactly because the Bard is many many things, probably the best they can do is truly a
class, and then use its subclasses to shift focus on different aspects. This would be the same as Druid subclasses, we have one focusing on spells and another focusing on wildshape, later there could be others focusing on healing, elemental powers, survival/natural lore, animal summoning/companions... If they make the Bard a class, then we can leverage on subclasses to make a Fighter-type Bard, a roguish bard, an arcane spellcasting machine Bard, a druidic Bard, a wandering minstrel Bard, a sage Bard, a healing Bard... there will be plenty of options for everybody taste, and everybody's personal view on what a Bard is or should never be.
I suppose you could do something similar also with a "build a Bard using different mechanical elements" approach, i.e. take a Background, 3-4 feats, a couple of specific proficienies etc. But I am not convinced this would work for a lot of people who just want to play a Bard character since day one.