I doubt they'll add more songs :/
By essence Bard was supposed to be a buffer but with AST.....he became, like you said, a ranger. Excepct for a song that would lesser physical resistance (like the magic we have) I can't think of buff that would not impact AST's gameplay considering he is now the buffer :/
Please meet (BRD 50+ quests)
Guydelot, another fellow BRD
As you can see, BRD do no use their bows as harp, even when you sing a song in combat, you put your bow aside and take a harp out of....out of where excaclty ? ^^"
Plus, beside the 50 relic, I've never seen a bow with a harp as part of it (and even here, you don't use that part but take a real harp xD)