The problem is this as far as massacre being G-horn only, almost all of our other songs are a multi-tier song, with a new tier gained as you level. Massacre is the next logical elegy in the leveling path and consistency would have all bards getting it at about 95-97.

If you want to throw G-horn owners a bone I would suggest a Stun+damage spell, something like Sound Blast as a name and put its damage in the realm of Holy II.

2nd option, "Cacaphony", AoE Raise/Reraise III (consider it an upgrade to Hymnus, specific to the fully upgraded G-horn).

3rd option, "Rally", a song that is cast on a player who has just used a JA (including 2-hour) that expands the effect of the JA to the rest of the party for the remainder of the time-or cast just before the JA is used to MAKE the JA AoE.

4th option, "Bedlam" a song that makes the prey it is cast on run in terror from all party members (like the anima back in Prommy).

5th option, G-horn can hit alliance members with songs, and is the only instrument that can.

Any of those would seem fair to me.