They can make a new class called fencer, a class based around wielding a one handed piercing sword like estoc with no offhand.

Then split it into red mage and mystic knight. Red knight would be a tank revolving around precasting spells on the RDM's sword that would ignite after taking enough damage or hits (either or). Basically, pre-cast cure (no cast time but doesnt activate effect), fight mobs, take damage/number of hits/parries fulfilled, cure activates and casts on self instantly. The rdm can also gain self buffs that stack that once it reaches maximum (like maybe 3 stacks with each stack reducing cast time of next spell by 33%), turns a spell into instant cast. Like, gain full stack, free off hand casts fire on sword hand, ignites and aoe around the rdm for damage and enmity gain (flash/overpower equivalent). Fire for enmity, water cures for healing, stone for mitigation, etc.

Mystic knights would be similar to the rdm I mentioned, except fully focused on damage dealing and debuffs and have a harder time of gaining stacks for instant cast spellswords (using fencer attacks to gain stacks, cant get hit or parry to ignite spells)