Good question. Lever identification's a big part of the balancing/game health assessment process for a new champion. In Bard's case one of the key reasons he's got a passive with so many different effects is that it gives us a lot of control over his tuning by game time, which is particularly useful given the lack of damage otherwise on his kit.
Giant list of the main balancing levers we see for Bard below. This is just from memory, so there are almost certainly a few important ones I'm missing however. Which particular levers it's appropriate to use will depend heavily on what sort of problems Bard has, whether that's excessive strength at certain points in the game, lack of counterplay, lack of sufficient reward for skilled play, inability to roam sufficiently or whatever.
Passive:
- Chime spawn rate, XP and mana given, spawn locations (with potential variation over the course of the game), speed boost duration/speed, lifespan
- Meep damage, slow, AOE, max number, respawn rate
- Upgrade effects, order, number of chimes required per upgrade
Q
- Cooldown
- Stun duration
- Damage, AP ratio
- Mana cost (mainly relevant early)
- Missile width, speed, range, punchthrough range
- Diminished effect for one target (if needed)
- Modification of what qualifies as second target for stun purposes (all enemy units? Champs only? Terrain or not?)
- Slow duration, strength
W
- Healing base, growth, growth rate, AP ratio
- Mana cost
- Cooldown
- Shrine max number, lifespan, bonus for being stomped
- Shine lifespan (if needed)
- Speed boost duration, amount, decay rate
E
- Cooldown
- Strength of speed boost for allies
- Max range (if needed)
- Ability to be used along the edge of the map terrain in some spots to travel in lane or not
- Visibility to enemies, amount of vision granted to allies or enemies while using the tunnel (how much of a surprise should people coming through a tunnel be? How much vision of the destination should you have when using an enemy Bard's tunnel?)
R
- Cooldown
- Delay till impact
- Duration of warning visual effect given to allies versus enemies
- Cast range
- Radius on impact
- Stasis duration
- Type of target affected (could remove towers or epic monsters if unforseen problems)
Plus base stats of course. Most of the above can also be tuned by spell rank too, depending on when in the game problems arise.
There are also some minor levers as well that aren't likely to be as relevant (W cast range, or E mana cost for example) but could be used if the appropriate issue crops up (e.g. if Bard has too little or too much early mobility E mana cost might be a useful tuning point, unlikely to be significant otherwise though).
