Red Mage idea

[Alerith]

There is no reason the combo system cannot make RDM work. I'm going to use pre-existing spells and sword abilities just so I don't have to make them up.

Now, give RDM Fast Blade, Flat Blade, Savage Blade, Goring Blade and Rage of Halone.
Also, give RDM Fire, Blizzard, Aero, Stone and Thunder, and only the first Tier.

Now, make combos that require using weaponskills to augment the power of the otherwise low-level spells.

Spell > WS > Spell II > WS > Spell III


Example:

Blizzard starts a combo and Fast Blade combos with it. You use Fast Blade and you can then use Fire to combo further. But now, since you are using Fire as your third combo skill, Fire becomes a single target Fira. Then Flat Blade can combo off of that and (assuming we ever get combos into the 5th stage) you can then Combo Thunder, but since Thunder is your 5th combo, it upgrades to a single target Thundaga.

This would apply no matter what combo of spells you used. A Tier I spell would start a combo, then a preset sword WS would continue the combo, then you could cast any elemental spell again and it would be at Tier II (-ra) level. Then you would use another sword WS and close with one final elemental spell, which would be Tier III level. (-ga)

EDIT:

As there needs to be obvious drawbacks that keep a single job from being overpowered, doing combos like this would either have to have a fair recast timer depending on how powerful your spells got, or they would have to chew through MP.

To go a step further, make the spells for RDM simply elemental damage without any additional effects like the BLM and WHM versions, and make them insta-cast when used in a combo. Make the recast time harsher and harsher the more powerful you make them. (Using a combo resulting in Thundaga would put a large recast on Thunder, etc.)

[Duelle]

As there needs to be obvious drawbacks that keep a single job from being overpowered, doing combos like this would either have to have a fair recast timer depending on how powerful your spells got, or they would have to chew through MP.

Another option to consider is spell damage caps and scaling. A RDM's spells are obviously going to be weaker than a BLM's version of the same spells. I'm iffy on recasts and mana costs because I'm not too keen on dead periods, which are expected of a dedicated caster but not from someone in the front lines. Going OOM due to carelessness is different from going OOM by design, after all.