I was thinking a bit about the various methods of how 'jobs' have worked through the FF series, and how SE is stating that XIV will be more dependant on things like weapons, and will lose leveling pretty much altogether.
Thinking back about how this has worked for previous titles we might be able to see the direction which they are going. I first thought it'd be similar to IX, in the fact that you learn abilities based on the weapons you equip. While in that game the characters were set jobs, and couldn't change them, the abilities they learned were directly related to what they had equipped. For example, Vivi, a black mage, could learn a spell like Blizzard if he equipped the right staff. When he had that staff equipped, he could use the ability, but only when it was on. If he got rid of the staff, the ability would be gone. The good exception was points were added to a pool which would level up the ability until it was finally 'learned', and would stay even if you changed weapons. So perhaps we're going in this direction?
Yet another possible idea comes from FF II and VII, where in this case the characters have no real 'jobs', but instead have differing stats, and their abilities are determined by weapons, equipment, and in the case of VII materia. If XIV is going more along the lines of VII, we can have our equipment which determines our base stats, and then our abilities come from items we equip (which would probably be in the form of crystals, as per the teaser art we've seen). So say you've got a baseline character, say it is a Hume so we have 'normal' stats. You equip a staff on your character, its strength, vitality, dexterity and the like go down, meanwhile intelligence, mind, and MP go up. Now stats are like a black mage, but then you equip something like crystals where you can use and level up magical abilities, until it gets maxed out.
A final theory is based more soley on FF II, wherein abilities and stats are determined by usage of abilities and spells in battle. So far FF II has been the only game to use this system. In this system, depending on your behavior in battle, after the fight is over stats and abilities are boosted according to what you used. While FFXI has used this for combat and magic skills, it was a slow and poor system as it was. What could be a possibility is more of an extention, where you equip a polearm and when you fight you skill up with it (but hopefully at a much more meaningful rate than FFXI). The more you use it, the better you get with it, the more abilities it unlocks for you. Furthermore, if you are hit more in combat, or use more magic, things like max HP increase or max MP, based on your needs. Like if you came out of battle casting a lot of magic, the chance that max MP pool increasing is there.
Those are a few theories on how FFXIV's 'job' system might look to be shaping up, but does anyone else have any ideas on what might come to pass?