This is different than an ancient single player RPG. Get over the fact that you are going to be locked in. It's going to happen and all your idea does is make the system stupidly complex. Changing weapons = changing classes. Also, assigning many different weapon types to one class or job completely cuts the uniqueness of any future classes or jobs.
Wouldn't it be better to save weapons to future potential of new classes leading into jobs than trying to make them into an extension of your nostalgia towards previous installments in the series? Think about it.