An archer bard would be a great fit for the party.
- Nice selection of arcane spells to complement the sorcerer.
- Bard song helps the barbarian and rogue significantly (potentially the oracle as well, depending on the build).
- You'll bring consistent ranged damage that doesn't worry about SR.
- You cover all the knowledge skills nicely and can work with the rogue to make sure you hit all the important skills between the two of you.
What kind of Oracle and Sorcerer? An oracle of life vs an oracle of metal make for very different characters. The sorcerer spec, as well, has some effect.
I'd say a bard is always useful since the bonus to attack/damage to the barbarian and rogue will be more useful than most options (especially since you can still do stuff while singing). You can also grab a few utility party spells like Haste. Unfortunately as my party's bard has found, this can be a little less fun to play. You can spend your feats on light crossbow stuff to be useful in combat.
Your party is also lacking a traditional high AC and HP fighter type like a, well, Fighter or Paladin. You could also go way outside of normal and pick up a Maneuver Master or Tetori Monk and practically fly around the battle field taking weapons away or choke-holding spell casters. Just because you can.
Bards are also useful because they can use healing wands. A definite benefit given the fact that the Oracle is your only healer.
Inquisitor can fill the 'skill monkey' role, provide minor backup casting, and depending on your build be a major ranged or melee force in the party. Especially starting at 10th level; already have bane, multiple judgements, multiple teamwork feats, and very close to Greater bane.