Assassin job (full concept & artwork)

[Mimilu]

Assassin is not a FF job. That said, Thief (Rogue) and Ninja are the FF equivalents of Assassin, as evident by their historic actions: Sneak Attack/Jugulate/Trick Attack/Hide/Assassinate/Poison themed skills, etc. These 2 jobs specialize in attacking from the shadows, moving quickly, and using throwing daggers/shurikens/venoms, etc.

Assassins were in Tactics Advanced and Tactics A2 as a job Viera could play.
In those games, Assassins were one of the best jobs in the game, having some of the best stat growth for Vieras, access to both a melee or ranged weapon (katanas and great bows), inflict status effects, and had an auto-KO skill (granted, it had a low hit-rate but you could use abilities and buffs to make that a moot point).

People, before you suggest a new job, ask yourself: 1) is it a job from a previous game? If not, 2) does the job represent the fighting style of a major protagonist or major villain? 3) has the job idea already been dismissed by Yoshida (e.g. Necromancer)? If the answer to these 3 are NO, then it will never happen. All current FFXIV jobs are jobs from previous FF games, or are jobs that represent the fighting style of a major protagonist (Squall Leonheart's Dissida NT movelist = Gunbreaker).

There's no rule say people can't come up with job concepts for fun. Nor do I see the OP demanding SE make it an actual job. :B
Trying to make obscure or new jobs fit into XIV's format can be a fun way to flex your creative muscles for some.
(Not to mention, things Yoshi-P have said in the past have been proven to not be 100% set in stone).