Let Me Level up in 5.0

[BillyKaplan]

not have to design weapons below 50/60 multiplied by the amount of new jobs.

Here's a potential solution - design the weapons similar to how the Ala Mhigan earrings are. They're for level one, but item level 50. They get the job done and help you out until you hit 50, at which point their exp boost also expires, giving you no reason to keep using them as actual gear.
Have the new jobs' starting weapon be item level 1, but a high enough ilvl so that it will both last you until you reach the job's intended starting level, as well as be good enough of a start for people who didn't start at level 1.
Lore-wise they can either say you accessed the Soul Crystal's knowledge immediately like they did for all post-ARR jobs, or just tell you to go and train until you unlock it through actual leveling. Story-wise the difference will be non-existent, maybe a text bubble or two.

Although let it be said that even if we're given the option, I don't think they'll start at anything under 30 simply because of the class->job system. Maybe when that's fully abolished, which I think will still be a while.