I'd really like to see DRK be a tank class.
The only way I would be ok with it being a dps class is if it was a sword/shield dps form for GLD. (And yes, I think sword/shield would be an interesting setup for a melee dps class.)

There just arent enough classes that are good candidates for tank. There really needs to be a balance between the number of tank/dps/healer classes, so more tanks really should be added as other classes are.
Samurai and Dark Knight fit into the tank role best, imo.

Either way, Dark Knight is a KNIGHT, and should use a sword & shield like Leon, Cecil, Gafgarion, etc. Its the iconic FF Dark Knight.