if & that is 1 major "if" they added a class like this & if I get the OP's idea the "class would have every ability from all class-only skills (preaty much every class from ARR lv30 & under & only the skills that didn't require a job stone) the best I could see it being is a clunkly full-trinity as it might have: 1-2 full arrgo combo options, maybe a Tank stance, near dubbles of each basic heal, at lest both Tank & range dps carbuncle, unlimited MP due to Umbral Ice, 1 if not both of MCH's turrets... & so maybe DPS moves (some of which might cause resistance down to a dmg type) it would be a litteral NIGHTMARE to use unless it has a limit on what skills it has per "Role" it decided to be for the fight along with probably a different hotbar setup per "role"

Even IF it got that far I can't even begin to decide if it should even be ALLOWED into the cross-job pool

In theory a neat idea but... it would be a dev statistical nightmare they couldn't balance fights around as easy