Just a quick house rule that popped into my head today, a way to reflect the fact that you can swing a knife faster than a greatsword without throwing off the system balance.
You know how every 5th point of base attack bonus grants you an additional attack at an additional -5 penalty?. For example, when you go from a BAB progression of +10/+5 to +11/+6/+1. So in this variant, for the purpose of determining extra attacks of this type, light melee weapons give you +5 to your effective base attack bonus while large melee weapons give a -5 penalty. Nonproficiency penalties also count in this situation. Also, if after doing the math your effective BAB is 0 or less, you need to make a full attack or full-round charge action to attack in melee.
So: a level 1 fighter with a greatsword must make a full attack or full-round charge action to attack in melee. That same fighter gets two attacks with a short sword, as the +5 bonus raises his hypothetical BAB to +6/+1 and thus gives him an effective progression of +1/-4 instead of just +1. At level 11 that same fighter's effective progression is +11/+6 with a greatsword and +11/+6/+1/-4 with a short sword. If he instead attempts to use a large exotic weapon he's not proficient in (such as a guisarme) the additional -4 penalty reduces his hypothetical BAB to +2 and thus reduces his effective progression to a straight +11. Am I making any sense? This is supposed to tread the line between having a neglible effect on the game and screwing up weapon balance, and I think it succeeds.