that is actually a good usage of current job gauge system. and it is not unheard of for a job to have a unique skillset (blm has manashield/ward, rdm has vercure). that's why job comp exist. And if blue mage excels in skills variety, they could perhaps lower the potencies that way you can maybe run a 1Melee 1Ranged 2Caster comp with blue mage taking the mechanic usually meant for melee (just an example). we can make it like jack of all trades. perhaps it can be good for prog, or for speedrunning. you're not gonna use all the jobs on all the occassions/contents. blue mage as a normal job can totally happen had they work into it more.