The basic idea similar to Zonpa-Zippa, a player gains the ability to not only control a Cardian, but is able to craft a small army of them. One of the coolest concepts introduced by the campaign system of "old" Vanadiel was the generals that had multi-pets. Primarily, the summoner and Zonpa-Zippa, had between 2-5 pets helping them. This idea of a multi-pet job would be a new introduction to the Vanadiel world, as well as tying a "crafting-like" skill to a job performance. These two concept could make a "Manuister" and interesting job.
The "Manuister" unlike, say a Puppetmaster or Summoner, wouldn't be relying on a strong pet, but rather an small army of mediocre Cardians. The skill of crafting Cardians would be tied to ingredients/farming Cardian cards and a "Manuister" skill level. These two would limited just exactly how many Cardian can be "out" and how fast can a "Manuister" craft in the middle of battle.
I'm in favor of having several tiers of Cardians(as they are now 2,3,4,5,6,7,8,9, and 10 of a kind) having their own "crafting" timers. This way 2 would be fastest to make while 10's the longest. Material formulas might also change over the tiers as well as which a player wants, for example coins, cups, diamond, or spades?
In general, I think a natural limit of Cardians per "Manuister" level would be, if a Cardian is the same level as player(Cardian 9 of cups, player lvl 90) then only 1 is allowed "out". However, if a player chooses a lower level Cardian, they can produce more.
Also, being autonomous, I think there shouldn't be a pet command for Cardians. They should function as elemental Spirits,as a Summoners. That is simply attacking what attacks you, attacking what you attack, and buffing as needed.
Maybe with this job, we can finally put a "good" use to those cards in the auction house.