Bard having new tiers for some of their higher level songs is more plausible.
99% of songs cap so early I'm skeptic to believe there will be new tiers for songs.
There is at least one exception: Horde Lullaby II.
More details
HERE.
This will make reaching the 486 skill tier super easy and the 567 quite realistical too (it was currently reachable even now, just not worth it).
If I take my current 486 set and add +99 it becomes 585, which is more than enough to reach the 567 skill tier (I could actually remove something to favor some... macc maybe)
Thelast tier is 648 string skill. Looks like with mastery it will be reachable in theory, but not worth it because of gear you would have to sacrifice.
(let's see what happens with Empy+3 gear and +string though)