In addition to the ability to make "custom" jobs using skills from multiple classes, I see a lot of potential for them to introduce special weapons for classes that allow them access to special abilities to further increase their character, yet still be able to change back through a weapon choice.
For example, perhaps you level MRD to 50 using 2 handed Axe. At level 50 perhaps you start finding special MRD only Great Swords that give you DRK like abilities as you level with it? Then you can throw back on the old Axe and be MRD with more tank like DD focus, versus DRK with more DD but less survivability whenever you want.
They already have abilities in game that require specific weapons so that they can only be used by a certain class, like Speed Surge. So why not have a ability, like Summon Wyvern, that ties to a specific type of lance, or has to be earned via SP gained with a specific lance then requires Polearm, etc.