One of the iterations of d20 house rules that I've played around with had a somewhat more fluid initiative system. Your actions, or those of your opponent, could modify your initiative for the next round. For instance, taking electricity or cold damage could reduce your initiative in the following round, while a haste spell could grant a small bonus to your initiative each round while its duration lasted. Someone with a high enough initiative got to make an extra standard action, taking the rest of their actions in the same round at a much lower initiative count.
Using such a system, a light weapon might grant a +1 bonus (or some other number) to your fluid initiative, while using a two-handed weapon might inflict a -1 penalty to your fluid initiative.
In the end, I decided that most of the time changing initiatives was more work than it was worth, and went back to a more standard system. In some instances (where fights were primarily between a small number of men, with no monsters or magic) fluid initiative might be more useful...