I actually had this conversation yesterday. I feel like, at the very least, we could have all class abilities in any dungeon. I.e. all abilities 1-50 except the job ones. This would limit the number of skills they need to scale, but also provide Dragoons more to do at level 25 and below than Heavy Thurst -> Impulse Drive ad nauseum (just an example.)

That said, I'm also totally fine with having all abilities, and would prefer it. I don't think balance is important at all in lower level content, aside from tanks still being able to hold aggro. But with the enmity buffs from 4.0, holding aggro is easier then ever. However, I feel like granting the class skills would at least make dungeons like Sastasha bearable.

Red Mage is actually a good example of this - by the time you hit level 10, you have all the spells necessary to utilize Dualcast, and by 30 you have your full spell ST spell repertoire except Verholy and Verflare. This makes Red Mage very enjoyable at lower levels, as you don't feel nearly as gimped.

EDIT: Red Mages don't get their oGCDs until later, but as far as GCD skills go, my point stands. Didn't want to get "gotcha'd" on that point.