Let's see...
-HD and BAB don't match up, in PF Poor BAB has d6 HD, Average BAB has d8, and Good BAB has d10, the only exceptions being Barbarian(d12) and Barbarian PrC like the Rage Prophet(d10 with average BAB).
FFd20 generally follows this rule, too... so I'd suggest giving it either average BAB, or d6 HD.
-Saves: The progression is weird, it doesn't really fit with anything that exists in PF or FFd20 aside from how a 20 level PrC might look... You can just give them Monk Saves, but that with all the features and 7th level casting might be overkill.
As odd as this may sound, you could give them all poor saves, at 1st level they can then choose to recalculate one 'Good' Save each morning, as they progress it eventually gives them 2, and at capstone they're all good saves? I dunno.
-Armor Proficiencies: Cloth armor is no longer a thing, so you may want to remove that.
-Spells: Not sure why they have 7th level casting, I think capping out at 6th level would work fine... still a lot to work with, and I don't think they should end up with better magic than the Red Mage, who is also a 'Jack of all Trades' sort of class.
Given it's fluctuating spell list, and very 'on the fly' feel, I think Charisma would make more sense as the casting stat. And rolling for which spell list you get, while interesting, takes freedom from the player... since they have full access at 1st level, and cannot use scrolls, simply letting them pick and choose from any other Spell Lists would be fine. Except Blue Mage, since there magic does not work like normal. Also a little wary on Summoner... but less so than Blue Mage.
-Limit Breaks: Onion Flurry, the progression is a bit off, might work better as two attacks, with one additional attack for every 4 class levels. Making it all run off one attack roll is a bit odd, especially as a Poor BAB class, and if it's one attack does that mean the damage is all pooled together? Or would DR and other factors apply multiple times?
Brandish the Elements: You should note that this still costs MP as normal for the spells chosen, and that he can apply his own Metamagic Feats to the spells he casts from an ally's list.
-Jack of All Trades: Fitting for what the class does, on this note since i missed it at the top, the class should probably have 4+Int skills. Normally I'd say 6-8 since it's a very flexible class, but it has soooo many other features that 4+Int should really be the limit.
-Self Taught: Armor Proficiencies dooon't really work that way, you HAVE to have Light to get Medium, you HAVE to have Medium to get Heavy... this is a bit of an oddball so breaking that isn't so weird, I guess, but it doesn't quite sit right with me. Not sure how I'd fix it, maybe progressive proficiency with light -> medium -> heavy armor and gaining a couple other weapons along the way?
-Way of the Onion: Ah here we go, Talents!
--Weapon Bond: Straightforward, simple, on par with other similar talents from other classes.
--Arcane Insight: Good talent, this seems to be their only way of gaining spells known outside of leveling up though... so I'd say take a note from Spell Blending(Red Mage Arcana). First time you get it, you gain Spell Proficiency and one Spell of up to the max level you know, or two of up to one level BELOW the max level you know. You can take it more after that, but you just get more spells, nothing else happens with Spell Proficiency.
--Feats: Bit better than most Talents since the Feats they have access to are VERY limited, but the class as a whole is about flexibility. It's strong, but I guess not overly so.
--Core Training: Intriguing concept, fits for the 'Everyman' sort of class, I like it... though when in DnD the only similar things that gave +1 to a stat were EPIC feats, it's a little wary.
--Arcane Onion: Cantrips? Huh, guess I didn't see mention of those earlier... uh, while they are Infinite use this feels like a very weak talent, so I'd say you take it once to get 4 Cantrips, with one more known for every 3 levels after 1st, as the Red Mage does iirc.
--Fruitful Presence: Haven't read Lucky yet, but I get the gist. This is a decent choice to have, I like it.
-Limit mimic: Uhhhhh..... this one is a bit much, being able to permanently copy a Limit is really powerful given there's the Mime PrC who can only do so for 1 round after seeing them used. For this? I'd suggest have it work like Brandishing the Elements, 1/day they can use someone else's Limit Break within 60ft, with one additional use per day at 9th and 15th level.
-Lucky: About what I expected, looks good.
-Hidden Potential: Another set of Talents, I always love these kinds of features.
--Onion Combat: Hmm, straight up boosting BAB is a bit of an odd move, especially with the progression... this basically turns it into an Average BAB class by 20th level, but that's very VERY strong. It requires Weapon Bond, right? So perhaps the way to go is they gain a scaling Class bonus to Attack rolls with said weapon, rather than actually increasing their BASE Attack Bonus.
--Indomitable/Sharp Instincts/Mettle: Aaah, I see, if you took each of these once you'd end up with all good saves effectively? That said, the progression on Saves is still weird.
--Skillful: And here's the fix for Skill Points, in your own hands to take. That in mind keep it at 2+Int, but increasing Int gives retroactive Skill Points so this should work the same. If you take this at 12th level, you get 12 skill points and 1 extra per level after.
--Ominous Presence: I like this, though honestly I think it'd be fine only costing 1 use of Lucky.
--Plagiarize: Aaaah, and here we have the other way to learn spells, an interesting one too. I still think Blue Magic and MAYBE Summoning should be the only spells an Onion Knight can't quite copy.
-Onion Mastery: Wow... LOTS of Talents, this is basically 'build your own class' >.>
--Overwhelming Spirit: Oooooh, I like this one, a lot. Doesn't seem OP or anything, just a really nice ability to have.
--Onion's Might: I'd say limit Wisdom to Damage with weapons he has Weapon Focus with, and adding half your level to spell damage/healing... I guess they don't have any Mageries to boost them so it isn't THAT bad.
--Tenacity: Very useful, you might wanna note that while it is a Free action it can be performed even when normally the Status effect would disallow those, such as petrification or Dominate.
--Onion Casting: Given how powerful some Metamagic Feats can be, this should probably also be something like 1/day/6 levels(3 max). There's ones like Chain Spell, or the +6 MP one that makes any spell last 24 hours.
--Improved Weapon Bond: Getting both feats at once seems a bit top-heavy, since these features start at 7th level and the last one is at 19th... you get Weapon Spec, then Greater Focus if 15th+, then Greater Spec for 19th+?
--Skill Savvy: I think disallowing it in combat is a little odd, but that might just tempt people to only use it on Tumble checks and whatnot so your mileage may vary.
--Onion Oversoul: Haven't read the ability it mentions yet, but I can see it at 8th level so you might want to add a minimum level of 10th to this one.
-Onion Spirit: I am wary on this one, the mechanics on adding classes to the list is hazy, but overall the feel is kind of cool. Just to clarify, when using this you only copy ONE feature at a time, right?
-Onion of All Trades: Useful, though mid-late game it's hard to say how often this will actually come in handy. I am reminded of a pair of Human feats from PF, Improvisation and Improved Improvisation... my thoughts are maybe Onion Knight gets Improv for free at 1st level (regardless of race) and Improved Improv at 10th level? This makes their 'untrained' checks a bit better than base ability modifiers.
http://www.d20pfsrd.com/feats/racial-fea...tion-human
http://www.d20pfsrd.com/feats/racial-fea...tion-human
-Limit Fusion: Aah, this one sort of clashes with my suggestion on Limit Mimic... but maybe it could just run off two available Limit Breaks within 60ft...? I dunno.
-Push the Limit: Some Limit Breaks this could really change how action economy works... and allow multiple Limit Breaks to be used in one turn. While my suggestion doesn't eliminate this possibility, what if it was the X/day you get to reduce the action required by 1 step? Full-Round -> Standard -> Move -> Swift -> Free...?
-Assumed Destiny
--Onion Weapon: Ah, this is where Greater Weapon Spec comes in... still, I think getting Standard Damage + 4(greater spec) + Wis mod + 1/2 level... even if it's poor BAB and only gets 2 attacks normally, this is a lot more damage per strike than a Fighter would normally be able to pull off.
--Onion Memory: This is a wee bit strong, Red Mage gets a similar ability for ONE spell, at 19th level... so this could use some tweaking.
-Onion Artisan: I'd say alter the wording for 'even when distractions would disallow', since there are some skills you can never take a 10 on... and never should be able to.
-Onion Aura: Nice buff, has some uses, can't be abused too much due to duration... altogether nice, if a little late to the game.
-Soul of the Onion: Oh jeezus... this is way too good, man, I can't really think of a way to fix it... maybe they can add ONE Ex or Su ability to their list each morning...? Even that's way too much.
Overall? This class has too much stuff, even if you have to spend features to get better BAB, Skills, and Saves... it has a Talent to choose every level from 5+ And it still gets a ton of features besides those. I like the concept, but you can almost outdo most classes at their main job and still be better than them at secondary jobs if built right.
