Kannik
Legend
This may seem like a somewhat obvious possibility, but I don't think I've seen it proposed before.
Rather than 'force' a character to wait until their BAB is 6+ to make a second attack (or more), simply allow them to do so at any time, with the usual -5 penalty, up to the maximum 5 attacks per round. This means for lower level characters, some of their attacks will be at serious penalties to hit. Any attack that would start at a negative BAB does not count a natrual 20 as an automatic hit.
Impact: only things with really low ACs would have to worry about this at lower character levels. But there's no real reason to force a character to wait and suddenly gain the ability to make that second attack. If someone at 1st level wants to try to make 5 attacks, well. Knock themselves out. Which they may just do.
Kannik
Rather than 'force' a character to wait until their BAB is 6+ to make a second attack (or more), simply allow them to do so at any time, with the usual -5 penalty, up to the maximum 5 attacks per round. This means for lower level characters, some of their attacks will be at serious penalties to hit. Any attack that would start at a negative BAB does not count a natrual 20 as an automatic hit.
Impact: only things with really low ACs would have to worry about this at lower character levels. But there's no real reason to force a character to wait and suddenly gain the ability to make that second attack. If someone at 1st level wants to try to make 5 attacks, well. Knock themselves out. Which they may just do.

Kannik