Thief is a melee DPS job branching off of rogue at level 30 that excels at unconventional tactics to gain an advantage over enemies. It specializes in alternating melee attacks and debilitating ranged attacks (xbow used in animation) to weaken powerful foes for their party. They also offer utility in the form of increased drop rates on open world monsters through actions like "treasure hunter" for those looking to farm materials and massive spike damage through actions like "ambush and triple attack". Gear would be dexterity based as thieves share gear with ninjas whose tactics could be considered similar. The job gauge could be a pair of crossed daggers with a number between them to show treasure hunter stacks at 52, and three xbow bolt heads at 62.
Lore:
While the warrior of light had beaten back previous attempts by the Kobolds and Sahagin to summon their respective primals, the beastmen still posed a threat to the city-state of Limsa Lominsa. There was also no knowing what the Imperial garrison in Eastern La Noscea had in mind, and with the warrior of light abroad the admiralty was faced with a choice. Do they stand by their decision to make the Thieves guild to surrender their name and become a sanctioned guild, or was it time to bring back the experts in covert operations that even the Yellow Jackets or Maelstrom would be hard pressed to complete. Merlwyb Bloefhiswyn quickly penned two letters, one for Jacke of the rogues guild, and the other to be sent to the warrior of light that was off in foreign lands. To help aid in keeping this possibly distasteful decision under wraps, Jacke would be tasked with using the rogue's guild as a front for the newly reformed Thieves guild. Only the most experienced and trusted of rogues would be given the opportunity to train as thieves, and that included the warrior of light if they even answered the summons. Regardless Limsa Lominsa needed someone to do the unsavory work even if it could mean controversy.
Actions:
30. Ambush: Grants precision to attacks performed behind an enemy increasing critical hit rate of actions performed from the rear by 30% for 12 seconds.
Cooldown: 60 seconds.
35. Acid Bolt: Fire a bolt coated in acid from your xbow to weaken the targets defense by 7%. Deals 100 potency and defense down lasts for 10 seconds.
Cooldown: 20 seconds.
40. Bloody Bolt: A barbed xbow bolt that drains the hp of the target. 100 potency attack that drains 50% of damage dealt.
Cooldown: 20 seconds.
45. Quelling Bolt: Thunder element charged bolt that causes a paralysis effect. Deals 100 potency while also inflicting "paralysis". paralysis: causes a target's attacks to be interrupted for 7 seconds.
Cooldown: 20 seconds.
50. Perfect Dodge: Allows you to dodge all melee, ranged and magic attacks for 30 seconds. Also resets the cooldown on Acid, Bloody, and Quelling Bolt.
Cooldown: 180 seconds.
52. Treasure Hunter: Adds "Treasure Hunter (TH)" stacks to a monster increasing both the drop rate and quality of materials dropped. Can stack of to 11 times and stacks are added by auto attacks while under the affect of "Treasure Hunter (TH)". Has no affect on bosses whose loot comes from chests. Duration 30 seconds.
Cooldown: 120 seconds.
54. Evisceration: Delivers an attack comprised of five 20 potency hits for 100 potency total with each individual attack having a chance to critical. Combo action: gust slash. Combo potency: 50 potency per individual hit for 250 potency total.
56. Accomplice: Steals 50% of enmity currently generated by target party member.
Cooldown: 60 seconds.
58. Triple Attack: Triple the number of strikes for your next attack. Duration: 10 seconds.
Cooldown: 90 seconds.
60. Mandalic Stab: Deliver a 450 potency attack and reducing enmity by half.
Cooldown: 60 seconds.
62. Collaborator: Increases critical chance of next action performed to 100% and transfers the thief's current enmity when striking an enemy from behind a party member. Enmity is transferred to the party member the thief is behind.
Cooldown: 60 seconds.
68. Trueflight: Deliver an attack for 480 potency. Only available when the Marksmanship gauge has 1 or more stacks.
Cooldown: 50 seconds.
70. Last Stand: Expends all Marksmanship stacks to deal massive damage while applying the effects of all three bolts. 200 potency per stack for a total of 600 potency.
Cooldown: 100 seconds.
Traits:
62. Marksmanship: Unlocks the marksmanship gauge (Three xbow bolts added under crossed daggers) allowing for the building of stacks for the use of powerful attacks. Acid, Bloody, and Quelling bolts each add 1 stack to the gauge. Increases potency of bolts to 120.
64. Despoil: Treasure hunter stacks also increase the the rate for a monster to be critically hit by 0.5% per stack.
66: Enhanced Mug: Mug additional affect: grants 7 stacks of treasure hunter upon successfully landing Mug.
post script edit: I know that SE probably won't be adding Thief to FFXIV. This was more just for fun and to try brainstorming what it could be like.