Though if you did add Onion Knight, I'd say if you made it have some unique innates and equipment options it could potentially be interesting... Innate Two Swords + mage equips or something? I dunno, I'm not very good at coming up with that sort of concept. I'm also probably way too late to the party to just be randomly suggesting things to a 99.99999% complete mod and should just work on my own if I think I can suggest anything. >_>
Innate two swords + mage equips is...only really useful if you can set secondaries.
Now mage equips + innate two swords on a class that can set secondaries is a thought, sure, but...magic carrier with good equip options and not much of a skillset themselves has already been done by Squire and Geomancer (and kinda sorta Bard, Knight, and Mediator). And there's other, arguably more interesting innates if you want to tempt magic users (innate Short Charge has not been used. A class with some way of getting transparent easily would allow for perfect accuracy spells. There's some possibility for interesting downsides, like a class with start: innocent but otherwise freakish magic stats).
What exactly could WotL Dark Knights do? I could probably just look up the answer to that, I suppose...
Some stuff that's pretty doable--like WP*PA stuff. A few moves that aren't easily with current editing tools--like a Malak-style move that uses low faith, except it's PA based--but could be done with hex-editing. And a couple moves that would be a lot harder to implement).
To be honest, though, I'd be happy just using the theme of the class, and taking some liberties with it. Like...I'd love to have a move that people wanted to use with a nice simple formula like PA * 12. Magic has tons of these. The physical side has a couple, but they're bad (most can miss, most are range 1, most deal really bad damage, and they're all random singletarget damage). And the rest of physical is all PA*PA or PA*WP (nothing wrong with either of these, just they tend towards different optimizations than PA * const does). Using existing formula...Worker 8-style moves on a human sound pretty neat.