For those saying that because there are only 16 weapons, we won't get much Job-specific customization since there will be multiple jobs on each weapon, and that the weapons will model the Aeonic style of stats (Copy paste stats, Unique WS); keep in mind that they could always choose to add job custmization through augments, similar to the original Relic Weapons, but with a couple additional options. It doesn't even have to be right away, they can add an option later to tailor your weapon to a specific job. I don't think they will actually put that much detail into these weapons (I agree the Aeonic model seems likely) but their creativity lately has surprised me with how they designed Dynamis weapons, Ambuscade weapons, and even the Bonanza stuff. I wouldn't put it past SE to actually put forth some design effort into these arms as a way to really push certain jobs towards the whole role/niche thing they have been doing recently.
As an example, I think about weapons like Great Sword/Dark Ragnarok and I kind of shudder to think they give Rune Fencer ANOTHER DD weapon because it would be paired with WAR/DRK, maybe PLD on it. RUN already has an amazing tanking weapon in Epeolatry, and Lionheart is a fantastic DD sword, so I like that they made the Dynamis weapon a Regen/Pulse Niche one. I see examples like this one and think that SE actually does put some thought behind the weapons they make, so there's hope they won't be lazy with the approach with Prime weapons.