Speaking with my bard player and showing him the preview of the bard, we can understand wanting to have a more definitive version of what bardic knowledge is, instead of "backup knowledge skill," or "what the DM decides to tell you," and various points in between.
On the other hand, it was kind of nice having bardic knowledge to fall back on if the knowledge skills all failed. So, with that in mind, this was how my bard player and I would kind of like to see bardic knowledge potentially, maybe, pretty please, handled.
I don't think this would break anything, and would still give the bard the chance to to "catch" any knowledge information that might otherwise fall through the cracks:
Bardic Knowledge (Ex): You have picked up a wide variety of stories, songs, and pieces of information that enhance your general knowledge, and this affects your use of knowledge skills.
First, you may use any knowledge skill untrained, as if you had invested ranks in that skill. Your knowledge checks, untrained, may reveal information that can only be obtained normally with a check higher than the DC 10 for general knowledge.
Second, for any knowledge skill that you have invested ranks in, you receive a bonus of +3 to those checks, which stacks with the bonus you receive for knowledge skills being class skills.
Third, one time per day, before you gain any information from a check, you may reroll your knowledge check and choose which result you will use for the knowledge check.