[Hiazro]

Why lock engineers ? Same for runemage. Engineers machines are made through focus energy. If Korvana have mage studying magic. They have people to progress technology.

Why lock kensei ? Yes in context of great six it is said it is a oniga tradition. But that just the thing. Korvana don't know about oniga. They don't care if the tradition already exist elsewhere in a place they don't know about.
In the end it's a class about mastering the fight art of swords.
And there's so much more than just katanas that demands a great mastery.
Rapier, sabre, longsword, and more...
I don't see why they should wait to discover oniga to unlock that "chakra" on their brain to use swords better.

This is the same for Boxer.
This is the same for monks.
This is the same for black knights. (black winds is just void. The rest is big weapon and armor. Seriously.)

I'm just going to be responding to what I quoted above and leave the rest of the work to all of you.

So, the problem with all of these is that they'd be a walking contradiction with how Korvara having a zero interaction with the Great Six. Engineer - as the class presents themselves - feels very Karaten-y and that's where all their stuff originates, pretty much. How did all the robots and turrets come to Korvara? Magic may come from many sorts of places, but you don't suddenly progress to having all of that cool stuff out of nowhere. And Korvara lives in a - for the lack of a better word - different 'world'. It's very hard to have a picture of floating medical bots tending to your needs when you came from a place that has no concept of these things. Invocation and all of that Evoker stuff, maybe - but robots?

On the matter of Kensei, yes. It's a class about mastering the art of swordfighting, except with an emphasis on katana - it's literally the theme. I don't think rapiers, sabres, and what have you - needs you to have a promotion class or anything to use them. There's a reason that despite many swords sharing similarities with katana, you're hard-pressed to find them used the way that those guys in Oniga do. Sword fighting is a very diverse thing and people are going to fight with swords their own way. 

Same thing with Boxers, honestly - it's a Bellaux thing. You can punch people to kingdom come and whatnot. but where did the Geist Schritting come from? I can somewhat see how Monks could develop in Korvara, maybe they'll have a different name for Ki? I can't say the same with Black Knights, using the black wind requires training sessions that you'd have nowhere else other than a wandering black knight kind enough to teach them. The big weapon and armor is a big part of them, yes - but the black wind is the reason why they're called the "Black Knights".

I'm sorry if I sound like a purist, but I have no idea how to explain all of these if I were to see them when Korvara drops other than that they probably come from the Great Six - which is unlikely. I can see how other classes that has no specific origin and more open-ended explanation (i.e: Aquamancer, Tactician, etc) could work in Korvara, but the rest is just going to break immersion.