While I do like the idea of being able to level up two jobs at once, I don't see much use in re-introducing split jobs unless the devs are at the end of their design rope. If they reach a point where they just don't have any new good weapon concepts, then yeah reusing weapons would make a lot of sense.

What we need to do is make peace with the fact that if/when this happens, the alternative job fantasies will likely be extremely thin. I mean look at Scholar's entire existence compared to Summoner. So this idea would really just function as end-of-cycle fluff to entertain players with some variety.

At the moment, I do not see this being necessary. I think there may be just enough design space left for a hammer green mage tank, a blitzballer DPS, and possibly a whip class. I could also see the devs try to eek out a fourth caster somehow. So I think we are at least one more expansion away from reusing job assets, if that ever happens.

I do have to say that as far as OPs proposed job alternatives, only BST seems relevant, and wouldn't need axes or bows when it could have a whip. I don't think pet jobs are happening, and even if they were, the old dragoon class would probably be wrapped up into BST. And PUP seems to have been mostly appropriated to MCH, which I think is flavorfully just much better. In short, I don't really see these concepts as being a strong draw for revisiting dual classes.