I highly doubt that they'd make a new job require 50 in the class and 30/15 in two other jobs (it doesn't even make sense for Death Knight to have CNJ anyways) since it would simply cause the character to skip right through any and all class story because they'd qualify for it right off the bat. If it were stronger than the other jobs (which it would kind of need to be to justify the increased requirements), it would render the other jobs for the class redundant (important note: class can't be rendered redundant because you still keep all traits and abilities from your class when you get a job; there are only 5 job abilities and they are unique to the job) while also potentially rendering other jobs in the same role redundant due to the increase effectiveness.
It's much more likely that, if the devs add new jobs without adding a new class, they'll simply tack it on to an existing class while changing the subclass requirement (e.g. DRK off of GLA requires 30 GLA and 15 LNC).
The problem with this, however, is that a class really needs to be built around the central idea of being able to branch into two separate roles at level 30. GLA doesn't have enough attacks to really create a compelling playstyle at 50, much less 30 (it's got a single t3 combo, a t2 combo that restores MP, a stun, and a block retribution attack), while also having all but one of the abilities in the PLD CD suite attached to the class instead of the job (which could *potentially* be turned into DPS CDs). It would take an incredible amount of revision of the class to make GLA into a class that could serve as a viable springboard for a DPS job.
The only reason that ACN can branch into jobs that fill 2 separate role in a viable and compelling manner is that it was built to do so: the base class is a DPS that gets a heal, a combat rez, and a DPS mechanic that can be quickly and easily coopted to become healing functionality (the pets) that also turns what used to be DPS CDs into healing CDs (Rouse, Eos and Selene's "special attacks" compared to the other summons'). The only things that SCH had to bring were a cleanse, a big heal, and an AoE heal.
DPS needs a relatively complex rotation and a substantial suite of DPS CDs (it could be argued that they also need some kind of enmity reduction capability, but MNK doesn't have it even though everything else does); tanks need a substantial suite of survivability CDs, a relatively simple rotation, and a tank stance (provided by the job); healers need a big heal, a maintenance heal, an AoE heal, a cleanse, and a rez. The class/jobs will have *more* than that, but those are basically the minimums for them to be balanced and compelling (it doesn't matter if it's balanced if it's too boring/hard/easy to play; it doesn't matter how fun it is to play if it's broken). As such, some classes just don't have what they need in order to turn into a different role: ARC and THM aren't going to be anything other than DPS, GLA isn't going to be anything other than a tank (with the 2.1 changes, the same could potentially be said for MRD with the buffs to the CD suite), and CNJ isn't going to be anything other than a healer. For all of those, the roles for the class are simply too heavily ingrained into the class itself.
It's because of this that the general consensus is that they'll most likely add new jobs along with new classes (as a further "benefit", a new class also prevents players from instantly reaching the end of a given job because they leveled up the base class months before). It's not *impossible*, since the devs can break whatever rules they have said or we interpreted that they have, but it's damned unlikely. Even those 3 classes that *could* act as springboards for jobs with different roles (LNC and PGL into tank jobs; MRD into a DPS job), there are still problems inasmuch as the classes already bring suites from their class that would be exceptional capabilities in their potential other roles (MRD has *way* more tank CDs than any other DPS and PGL/LNC both have a lot more +DPS than any of the tanks do).