I think it should remain as a completely random weapon drop, as if it was guaranteed to be one of the 8 classes present, A: it incentivizes people to play undergeared classes more (eg if I have the WHM weapon, I'd try to not play WHM at all costs, to raise the chance of getting a weapon for another healer), and B: it raises the chance of someone getting a dupe weapon they can't take (eg, if I ignore A, and play WHM anyway cos it's my most geared, it raises the chance of another WHM weapon dropping)

Instead, I think a fair solution would be this: given that the last fight used to drop the chestpiece coffer, and that's been moved down to the 3rd fight, there's a gap left in the loot where that chestpiece used to be. So, have the 4th fight drop TWO random weapons, plus a coffer, with the only rule being that the two random weapons cannot be the same one. So, NIN plus NIN drops cannot occur, but NIN + DNC can, even if a NIN or DNC is not in the party. This effectively doubles the chance that you get a random weapon that someone can use, but keeps some level of RNG to it. Maybe you get WHM AST drops and it's sad yes, but that's the luck of the draw.

This would also help those kind players who enjoy helping PFs get their first clear, as twice as many random-weapons dropping means faster gearing up alt roles to flex to