To allow more interesting builds change it so that any combination of non full bab classes all consider each other for bab progression, so for example:
Any combination of classes with 3/4 BAB progression would always get 2 attack per round by level 8. A level 1 rogue 1 druid 1 bard 1 cleric would have 3 base AB.
Same with 2/4 BAB progression. So for example a 1 wizard 3 astro 3 rogue 1 druid would have a BAB of 5.
The BAB would still progress with the first level always being 0 BAB or 3rd level being 0 BAB for 2/4 classes but it would take every class into account rather then having a separate BAB counter for each class, opening up all kinds of funky builds.