How to implement Red Mage:
Give RDM 3 "modes" that let's it swap role to DPS, Tank or Healer. This role must be chosen before queuing for dungeons etc. just like all other jobs have to and also make them unable to change role in battle and in a dungeon.
The other abilities would always stay the same, but would do different things based on what role you are in. For example more damage in DPS role, enmity gain in Tank role and an AoE heal in Healer mode.
Finally, you could let it cross-class every skill currently cross-classable. Of course with the same limit to them as other jobs currently have.. 5 of them at once, or something, is it?