Iterative Attacks have always felt weird and clunky to me. The forced -5, -10, and -15 makes those later iterations almost pointless; they rarely hit.
What if, instead of stacking a -5 on each extra iteration, after each five you can take a -2 on all attacks, similar to the way rapid shot works for ranged weapons, and get extra attacks?
So after BAB 6, you can either make a regular attack, or take a -2 on both. After 11, you can do one at full, two at -2, or three at -4.