Muskateer (whenever it's introduced) doesn't branch off into a Chemist.
It's a bit off the norm, but I think it would be great.
As for why Muskateer? Well, in FFT, Chemists were (to my knowledge) the only generic class that could equip a gun naturally. So in my mind, I always considered the gun to be the Chemist's trademark of sorts.
The only problem I could foresee is the Muskateer will without a doubt be a DD class, and it's hard to justify teaching a class that is all about shooting monsters in the face a healing skill, but I ink they can swing it with certain job skills.
Something in a similar vain to Defiance, where you activate it once and you're fine, where the effect drastically reduces the cooldown for all the items while droping your strength a bunch would be great. So for example, you could toss an X potion every 10 seconds or so, and maybe later you can get a skill like Salve to make any particular potion affect the entire party. And let's not forget about status potions too! Wanna silence an enemy? Go for it.
But relying on items to heal may not sound like a good idea, or cheap, so maybe toss them a skill like Healing bullets. It makes me think of Monster Hunter, where bowgunners had healing shells that pelt their allies with health. Though if we're up to me (like the rest of this topic isn't) it would be that strong, to encourage potion usage, though not necessarily require it.
Maybe they should introduce item levels to potions too (if they don't have them now that is). The reason I say this is so you don't have a fresh 30 Chemist spamming Elixers. Just an idea.
Without a doubt, if they do introduce Chemist in the way I outlined, it would be the Rich Man's Job. It would make Alchemist insanely required and in demand.
Alternatively, they can give them skills that mimic items too. Although I would want to keep the skill that would decrease item cooldowns, maybe they could get a Remedy skill or a Pheonix Down like skill (Pheonix Pinion?).
I don't know, the things I think of when I should be taking notes.