I think this is the right approach to future job predictions, as I have observed in other threads that FFXIV job design is very centered around having a unique weapon template and animations for each job, each job has a subset of weapon terms that only it gets to use (with a very small number of exceptions, most of which come from very early ARR), and weapon glamours are a core appeal of gameplay.
My best guesses for future jobs weapons:
* Blitzball - Likely accompanying a slight Scouting rework. Extremely likely after GNB because of being *the* archetypal FFX job with three characters and a minigame to pull ideas from. And we don't have an "athlete" job archetype, so the design space is generally untouched.
* Whip - Flails, Scourges, Chains, Tails, Hooks, Claws, Morningstars -- just a very open design space at the moment. Could be dual whips. Could have a weight at the end. Something whippy is very likely. I also suspect this would be the Trainer/BST job, because why else would it have not been used for BLU when they had the chance?
* Mallet - I don't think we will get a Greathammer/Warhammer job since that maps too cleanly onto WAR, but I do think we could get a slightly lighter two-handed mallet/gavel (mace, sledge, etc.) job for a Green Mage. Make the head of the hammer the a crystal focus as a kind of inverse to RDM's sword-pommel focus.
I don't think we are as likely to get a dual-wield sword job so much as a dual-wield Judge job, where the staves can have any combination of swords, axes, maces, etc. etc. We already have five sword jobs, and I don't expect the devs will waste an opportunity to feature more unique weapon designs AND a popular FFXII archetype on just swords. This also gels with the idea that Judge really wants to be a tank job, and giving it a wider variety of options than swords would help distinguish it more. The only obstacle I see to for Judge is the weapon terms it would use, but I think like Gunbreaker (Manatrigger) and Reaper (Crux) they could scrape together a few unique names.
Other throwaway ideas: Reflector/Mirror Illusionist/Shaman Caster. Cannon Corsair/Alchemist Pranged.