Thief/Assassin/Rogue/Ninja Class

[Atreides]

Or take the traditional Final Fantasy approach and do Thief => Ninja

Thats not a Traditional approach, I can only think of one FF in the series where a Thief would turn into a Ninja.

[Kitru]

Thats not a Traditional approach, I can only think of one FF in the series where a Thief would turn into a Ninja.

That's mainly because there aren't direct class/job upgrades in most FFs. The upgrades are indirect in order to prevent turning any given job absolutely obsolete. With only a single exception (FFVI; Shadow and Locke), in every instance of an FF with both thief and ninja, the ninja either requires levels in thief (the Tactics games) and acts as an upgrade (similar stats but better) or shares a similar stat distribution that is simply better (thief is fast, evasive, and hits reasonably hard; ninja is faster, hard to hit, more durable, and hits harder).

The only reason that ninja is an explicit upgrade to thief in only 1 game is because, in that game (the first game), once you get ninja, you can no longer access thief. Because you still have access to said class, there should still be some reason for you to use said class (which is also why you see no significant ability overlap; there is some spell overlap, but only in cases where the categories are meant to have specific value: red mage overlaps with both white mage and black mage but renders neither redundant by not being able to access higher level spells; summoner has spells that are significantly stronger than black mage spells but does not render black mage redundant thanks to high mp costs preventing the same prevalence of use).

Furthermore, thief and ninja fulfill both the same niche (rapid stealthy attacker using light weapons) while using what amount to the same weapons (ninja blades are effectively upgraded daggers restricted only to ninja).

The only way that we're going to get both thief and ninja (in recognizable forms) is if thief is the class and ninja is the job. We're not going to get 2 classes because they would be too similar, and we're not going to be getting them as 2 jobs off of the same class because they're both DPS jobs (and the 5 abilities you get from a job are not sufficient to significantly distinguish the gameplay of one from the other; you'd end up with 2 jobs that are 95% the same, which basically renders one redundant and, as such, developmentally inefficient).