They could also remove daggers from GLA completely for launch. Every time one of these kinds of questions were asked they always used the "placeholders" excuse. They could be hiding the fact that they are making GLA Longsword/Broadsword only for launch or later.
My guess is that they will just make some of them [Thief] Arms and some will be Gladiator Arms. I can't imagine they don't make Thief a Class or Job since Yoshi-P has already said they need a class like this as well as it being a staple FF job.