It seems I need to repeat a lot of the stuff I have been saying over at the WotC Future Releases forum...
First off, to address the main topic, I really have no idea whether the Monk should be a Ki Striker, a Ki Controller, or a Ki Defender. Honestly, I would prefer it is the traditional D&D "Monk" was given a different name this time around, since I really want to see a Divine class based on the image of the wandering Buddhist monk seen throughout eastern pop-culture. I doubt that will happen, though, so I'll just use the classic name for the class for now. However, since the Monk is almost certainly going to be a Ki class, rather than a Martial class, I have no doubt that it would make a good Controller if WotC chose to do go that route, and even if it is a Striker or Defender I imagine it will be a little Controller-like.
Before I talk about the specifics of classes, I want to say that I think Ki will essentially be the flashier and more energetic version of Martial. It will probably be based on weapon attacks and the like, augmented by supernatural effects and flavor. Comparisons to fighting games like the
Street Fighter series or anime like
Dragonball are probably apt. As such, it will probably more closely resemble the good parts of Tome of Battle than it will look like any 3E classes.
As I mentioned above, I think a Ki Defender or Striker version of the Monk is fairly self-explanatory, but I also see it working perfectly well as a Controller. Just because a Controller works in melee doesn't mean that it will be "sticky" like a Defender, after all. For example, look at the Setting Sun discipline from the 3E Tome of Battle (one of my favorite disciplines from that book). In addition to countless throwing moves designed to hurt foes and move them around the battlefield (away from the user), it has one move, Ballista Throw, which a PC throws an enemy through a group of other enemies like a line attack. Such a set of moves would work beautifully as the basis for one build of a Ki Controller Monk (the "soft-style" build, maybe?). Another build (the "hard-style" build) of such a Monk could be built on Hadoken-like ranged energy attacks, ranged punches, and blows designed to strike the "ki" of the foe and cripple him (I am a big fan of the Hyuga clan from the
Naruto series). In other words, it would be a "melee" controller designed to stay mobile while pushing and throwing enemies around, using special ranged attacks, and occasionally using more sticky and debilitating moves. Maybe it could even send out a shockwave of ki or grab an enemy and swing him around in an arc as a close burst attack in order to have more area of effect moves.
Darn it, the more I write about the idea, the more I like the sound of it. Just when I was getting set on the Samurai being a Ki Controller, too... Oh, I should probably explain that one.
You see, I have been arguing on the WotC boards that the Samurai could make a pretty interesting Ki Controller (or at least, that is one good thing the Samurai could be). It could be a class capable of sending out ranged energy slashes from the blade of sword, performing supernatural Iaijutsu attacks that cut down every enemy in a wide close burst, use a bow to shoot arrows charged with ki that explode at range or seal an enemy's power, etc. Actually, this idea could be fused with the Controller Monk idea I mentioned above to make a great new version of the Swordsage (though they might need a new name thanks to the Swordmage).
As for a few other things...
Ninja: I don't see this as Ki. The shadow-assassin ninja is already perfectly covered by the Rogue, so if they make a dedicated Ninja class I want it to be an Arcane or maybe Elemental class dedicated to the most out-there stuff you will ever see a ninja do. Fire-breathing, frog-summoning, ninja-vanishing, doppleganger-using, tricking does into attacking a log duplicate, elemental magic attacks, true invisbility, etc.
Wu Jen: This is just a slight re-skin of the wizard, so I don't think it deserves its own class, and it certainly doesn't work as a Ki class. It might work as a new Arcane or Elemental class if it broke free from its 3E version, though.
Sohei: This could either be a Ki class or a renamed Paladin. The name quite literally means "warrior monk", and I think the traditional Sohei are basically ascetic monks who fought like Samurai, so it is probably a pretty good name for a Ki class, if a bit unknown and generic... I can't even remember what the 3E Sohei were like...
Shugenja: Rokugan-style Shugenja are not a Ki class, but instead would be Arcane or Elemental, just like my preferred Ninja. However, the Shugenja of history were indistinguishable from Monks and Sohei, and didn't even resemble Rokugan Shugenja... As long as it is distanced from the Rokugan style, it would be a good name for a Ki class or variant of another Ki class.
Samurai (again): The issue with the Samurai is that I can easily see a class named the Samurai as a Martial class that doesn't rely on the supernatural effects of Ki. Of course, a Samurai as Martial Defender seems a bit redundant (though a dedicated armored archer class would be cool), but as I mentioned elsewhere on these forums recently, I can see the Samurai as a Martial Controller counterpart to the Warlord, based on intimidation and fear effects.
Overall, most of the actual class concepts I imagine working as Ki are Swordsage/Monk/Samurai variants that fit into different roles and pop-culture archetypes. That one set of concepts is flexible enough to make a large number of classes out of. Trying to force other ideas into the Ki Power Source seems a bit unnecessary to me.