And material on the ninja have a lot to do with special, quasi-magical things. The ghost step "Turn invisible" or "turn ethereal" is about right in terms of the mythology associated with them. Yes, there's a lot of rogue there. But there's also the mystical powers. Sure, a "Rogue + Some new abilities like a Quivering Palm" could do it.
So could "Unarmed fighter + Mystical tricks" function for a kung-fu artist.
At the same time, a paladin could be achieved with: Base fighter guy + Lay on hands and this or that.
I don't see WotC taking the "Base + Stuff" approach.
Andy Collins,
in the Tome podcast, said that the class design philosophy for 4e is "What does this character Bring to the Table in combat that no one else can do at all". He specifically pointed at the monk and said "Well, he can jump around and climb on stuff. Well the wizard can fly; jumping around isn't so great. So aside from having a nice story, what's the point of playing a monk?"
Therefore,
each class introduced has to be able to do something that No One Else can do, at the same time as it fits into the role paradigm of "where you fit in the party". So the Monk, if it's going to
exist,
must do something that the other classes can't. Same with the Ninja. This is why enchantment was held for psionics, so that they'd have something the wizard can't do. Same with Necromancers, et al.
Dollars to donuts, I'll bet the monk will have stunning ability, and tricks like "Move six squares. You can make a base attack against three opponents in that line, and you do not receive OAs for it", because no one else can do things like that yet.