I like how you merge two classes' clothing into the next, but it kinda makes several classes look too close. It's more realistic, and it'll look quite nice in a game, but I'm afraid there might be confusion once you're actually playing, when you need to guess the class of an enemy, especially if they use another pallette for team colours. Chemist, doctor and engineer are too close for example. Same with alchemist and master surgeon
Also, the outfits are a little basic on the higher classes. I dunno, but I'd expect some really fancy armor on a champion, not a bleached knight. That brings me to the pallette. As mentioned, it kinda reminds me of Ragnarok, which isn't basically a pro, since that pallette is so overdone. Why not go with alot more vibrancy in the colours?
I really like the diagram, though I'm not sure I'd do it the same way. There's some fuzzy naming going on. Change "Hero" to something like "Rebel" perhaps? "Hawkeye" is too creative and doesn't fit in as a name, perhaps something like "Assassin" would be better fit. "Alchemist" of a "Chemist" upgrade, it doesn't seem to make sense after engineer. perhaps "Scientist"? I think you can drop the "Master" on "Master Surgeon" and the "Arch" on "Archmage" ("Wizard" would be even better), since those seem out of place. Why have a master surgeon if there isn't a regular surgeon? Why have an Archmage, if there isn't any normal mages? I'm not too keen on a priest being a combination of a chemist and a magician either. I'd go with a scheme where you have a basic "adventurer", which splits into "chemist", "archer", "warrior" and "cleric", since then you can make a "crusader" in between "cleric" and "warrior", and a "doctor" in between "cleric" and "chemist". In between those you'd have your "priest" and "bishop". Still not the best though, there's something inherently wrong with the hexagonal class system you have. I'd have to give it all a little more thought to come up with something decent though...