I would rather see a tangent to 2nd/3rd classes rather than a 4th job class.
For instance, I've been thinking of the job Pirate. And was thinking that Gravity could remake their WoE castles and have moats, shores, and other such things. Perhaps Pirates (like in Fire Emblem) can travel on the water, allowing for new and unique strategies (for instance having guild leader as pirate, and having them enter the castle through a different route).
Too bad that'd probably take too much coding and would be too game-changing for RO to implement. Would have been nice though. :< There's ninja. Why not pirate? Dual-wielding like an assassin, but can dual-wield both melee weapons and long-range (aka guns).
And also for Ninja/GS/etc. With 3rd classes I'm assuming these guys will be significantly at a disadvantage? Give them something new.