I believe that our sub abilities should be incorporated more into the job to help with variety.

Example: My Idea of rdm implementation
Job: Red Mage
Requirements: Fencer 30, CNJ 15, THM 15
Sub classes would obviously be cnj and thm

RDM lvl 30 ability: Elemental strike
-Delivers a non-elemental damage attack
Combo: Fire/Blizzard/Thunder/Cure/Aero/Stone
-Changes the element to element of prior combo spell, and does additional effects based on previous spell

It would work like this:
Fire -> Elemental Strike(ES) = Flame strike(pacification + fire damage)
Thunder -> ES = Thunder strike(stun + lightning damage
...etc, earth would give slow, ice would give paralyze, aero would give silence, water would give poison

The lvl 35 ability would be something for utility, idk about this

RDM lvl 40 would be a ws that finishes off a fencer 3 step combo

RDM Lvl 45 - Spell Blade (couldn't think of a name - SB for short)
This ability would allow access to a four step combo. The combo would be:
Spell -> ES -> spell -> SB
Depending on which two spells used, different effects would happen
Example
Aero -> ES = Windstrike + silence -> Aero -> SB = Spell Blade: Whirlwind(add effect dispelga)
or
Aero -> ES = Windstrike + silence -> water -> SB = Spell Blade: Typhoon(erases one debuff of all party members)
or
Thunder -> ES = Thunderstrike + stun -> Thunder -> SB = Spell Blade: Indignation(Lightning damage and AoE Stun, and/or always crits)

I could go on and on and on with rdm, but I won't, to many combos to think of. But I hope this illustrates how many options SE can pull off with the class system if done right, even in its current state. RDM would be a front line DD like everyone wanted it be with this set up, and get cool effects.

TLDR: There is alot of potential in incorporating the abilities of sub classes for a job into combos.