Perhaps you can use one additional combat feat per additional attack you may make. If you can make two attacks because you have at least a +6 BAB, then you can use two combat feats in a given round. This would reward the combat classes with the highest BAB, since they'll probably be the ones using these feats most. This might be too powerful though, as at 20th level you'll be able to use 4 per round. I haven't yet had a chance to play-test any of these feats, so I don't know how game changing they are.
