A 6th passive slot have been asked for many times, so i doubt that is something they will do. Special Weapon Effects are a sort of passive skill, in which case you have 7. Some of the Weapons have passive abilities that correspond to some of the actual passive abilities of that same weapon. I assume they chose 5 actual passive slots, because you can then either chose to have the passive of each of your active abilities (since basic doesnt come with a passive), or chose a more weapon general passive at the cost of one of the ability specific ones.
Regardless it is a legit suggestion and should probably go in the Feedback and Suggestion thread =)