This is kind of why I'm waiting to see the job system, since the all the Armory system did was remove class uniqueness. (for instance its not uncommon to be in a party where everybody has Cure II and Bloodbath) (or weapon-skills that can be used with any weapon, like Circle slash with a staff. lol)

Make skills over lv30 exclusive to their native class?

Kind of outrageous, I know but adding restrictions will force people to make better use of their main class's abilities instead of getting everything to 50 to become a super hybrid.

Maybe rise the slot cost of abilities from other classes?

Or just hope that the job system just solves everything lol.