First of all, the Mage's ability. Preventing cool down for fifteen seconds seems either really, REALLY underpowered, or really overpowered. It really depends on whether or not your other activated skills have long cool-down times. Negating cool down times by fifteen seconds for all enemies can be a massive game changer if your other skills have 20 or 30 second cool downs. If they're much longer than a minute or two, however, it quickly changes from really OP to really underwhelming. Assuming you're going with most classes having a lot of 10-30 second cool down skills/abilities, I would change it to maybe a "tag" feature, where you can tag one or two opponents and pause their cooldowns. It also fits more with the Monk motif I think, having to touch their opponent and "shutting down their chakra" or some other excuse.

The knight should have either a longer cool down, or change it from complete damage immunity to like, 50-80% immunity. Samurai could be OP if your stats hugely impact your damage and defense, same for the speed aspect since they'll be building speed (being a speed-oriented class). Maybe a flat stat bonus, or only a doubling of speed and attack? Dragon Tamer seems fine as long as it's for one encounter a day.

Beast Master is interesting, because his skill makes sense, but it's not necessarily believable. I don't think it would see much use because it would focus damage on their beasts, which they would probably have an attachment to. I could see there being a handful of default choices for your first beast companion, and that beast gives some kind of scaling passive bonus or skill? Summoner is fine, as long as there's some kind of limit for how long ago the dead can be revived. Resurrecting long-dead kings or warriors would be very OP.

Assassin should lose on of its two effects, ignoring defenses AND dealing all crits (I'm assuming deal 1.5 or 2x damage) would wreck everything, especially since they're a DPS class. Rogue is fine, but they're not really a "tank" class, I could see them having TONS of passives and a lot of utility though. Trickster is good, love their flavor. I'm mixed on Gunner. His ability would be really good against bosses with health lower than 25%, insta killing them. But they wouldn't be good against hordes of enemies. I would recommend having two different builds, a sniper and an assault version. Sniper gets the existing ability (Maybe tweaked to insta-kill either a single minion at full health or a boss at 10%), and the Assault getting some sort of area-attack that deals a single crit to every enemy? Or maybe their ability allows all of their attacks to automatically hit and crit for 15 seconds.

I dig the Tribesman as a tanky bait/controller. His aggro focus seems like a good idea, but his skill is a little underwhelming. Maybe some kind of defense buff to go along with it, or a slight debuff to enemies he hits with the skill? Archer is fine, but kind of faces the same problem as Gunner. Archer in this case seems geared towards area attacks automatically, but should be able to specialize in either sniping or area. I'd give them an option similar to Gunner, with either an area ability or a singular one. Or maybe a buff, every target attack hits and crits for 15 seconds?

Mage classes have been covered. Magic Person (Not a Mage class?) seems a bit 'eh', they seem to be jack-of-all-trades, but also with a focus on healing? I don't know, I wouldn't want to play one personally. The Bard seems fine, but maybe a passive ability instead that grants a boost to SP and MP regeneration instead of a flat bonus once? Priest seems a lot like Black Mage, with a summoning class skill and the ability to bind enemies. The biggest difference is that Priests seem to focus more on disorienting or disabling enemies, while Black Mages focus on draining or killing them. That's fine, but their power should also be similar to Black Mage then, summoning a spirit which deals Holy damage which scales with the user.

I most want to play as Priest, so I'm really interested in anything involving the class. Gunner comes as a close second, I might consider a second character just to try both. 

I'd also consider losing Intelligence as a stat, since Intelligence is really the only thing that the player IS. An intelligence in a faux-body, stuck in an electronic world.