As much as I'd like a sub-job system, there'd always be inherent balance issues, even there weren't any abilities that directly boosted DPS available.

IE, BLM could give any job access to Aetherial Manipulation, Manaward, or Convert, but the first of the three would likely be the most popular due to allowing you to reposition before or after mechanics faster so you can DPS longer.

I'd take slightly more imbalanced classes for the sake of having meaningful options available, though.