Instruments
Instruments wouldn't be actual equipment items, but instead function like stances or elementalist attuements. Instead of two weapon sets, you instead have two instrument sets and three "Tempos." You wouldn't be able to change your main-hand weapon in combat.
For actual instruments, I'm thinking one for each race:
Charr Wardrum: Thundering marches are the order of the day, with emphasis on combat boons and control.
Human Lute: A variety of songs from the long history of humanity, with a mixture of support, damage and control.
Sylvari Reed Flute: Ethereal melodies with an emphasis on regeneration-oriented support and high single-target damage.
Asura Harmonizer: A mechanical device with the highest damage potential of the Bard's instruments.
Norn Warhorn: A massive Dolyak horn with songs that emphasize control and heavy condition infliction.
Songs
The three instrument-based skills would all be considered songs and function like chain attacks, so you can start with the introduction, then move to a refrain, then end with a cadence. You can then choose to start the song over again (if you wait for the cooldown to end) or move to a new song. The cooldown between each step of the song is short, while the cooldown of the entire song is quite longer (but this cooldown is only triggered when the song has ended).
Each song has a "Discord" option; if you switch to a new song during the Refrain or Cadence steps of the one you're playing, it can cause a specific effect, usually inflicting a condition or effect on nearby enemies. This ends the song, however, activating the longer cooldown.
For example, say you've selected the Wardrum instrument, granting you "March of the Khan-Ur" in your third slot. It's a Charr marching song, strong and resolute; definitely a tank/control song;
Introduction: The song starts without warning, raining fire upon the enemies of the Charr. This causes damage in a circular area around the Bard that inflicts Burning.
Refrain: A ring of Phantom Charr spears erupt from the ground, raised in tribute to the Khan-Ur. The spears cause damage instantly and remain in effect for a few seconds; enemies who attempt to leave the circle suffer the Crippled condition.
Cadence: Thundering to a dramatic close, the cadence grants Might to all nearby players.
Discord: Should the bard interrupt the "March of the Khan Ur", vengeful charr spirits draw forth and attack the Bard's target, inflicting the Chilled status. These spirits remain active until they've caused a set amount of damage.
Tempo:
Each Tempo would simply be a self-buff that alters your character, but you can obviously only have one active at a time;
Adagio- A slow, steady pace; grants Protection and immunity to Immobilize when active. Additionally, any Discord effects last longer or cause more damage.
Moderato - A moderate pace, neither too fast or too slow; when active, it reduces the cooldown on each phase of a song by a small amount, and the wait time on entire songs by about half.
Allegro - A quick pace, granting Swiftness and Fury. Additionally, all song Introductions grant Vigor to the bard and all nearby players.
