I like it. Class req's: Gladiator 30/Scholar 15 (Scholars have healing and dmg spells, so it fits more-or-less). That being said, I think RDM fits better as a DD. Give them DoTs and En-Element weapon enhancements to suit various circumstances.
I don't understand the idea of making Dark Knight a tank class. My first experience with the class comes from FFXI where they were straight DD that couldn't last 10 seconds with mob aggro, and I liked it that way. The stark counter of Warrior/White Mage being a protector vs. a Warrior/Black Mage being a destroyer was thematically beautiful.
Samurai, on the other hand, I would love to see as a tank. Being that they were the "knights" of feudal japan, the parallel to Paladins is obvious. Make them a DEX-based parry tank to give them a distinct flavor.