Paladin and Dark Knight already exist. If a Rune Fencer / Mystic Knight came out I'd want it to be DPS to separate people who want a Spellblade from people who want RDM to be a hybrid white magey thing. As a tank it would just be another sword tank ( we have 3 ) and another magic tank ( we have two, both use swords ).

I don't mean to shoot the idea down outright but a new tank needs to definitely not use a sword and Red Mage needs one clear job identity, not force two clashing ideals ( swordmage and healing support mage ) under one banner.