Like I mentioned already it depends on how SE would approach the situation, but based on their past MMOs they do like to play "favorites" (we shall not discuss this here as it's another thing entirely), but this is a new development team. So its hard to say how it would pan out. One thing to note is that all these jobs are from past FF games and each has a different set of abilities that make them unique. Some smart moves for SE to make are:

1) Plan ahead and have ALL your jobs listed (or just have way more than enough, I would say at least) this way you can plan content with jobs in mind

2) Make each job have a unique idea to bring to the party. It's fine if some abilities are similar its bound to happen, but imagination is the limit when making abilities. They should remember that no job concept is game breaking, its the strength and total combined effects of the abilities that make jobs overpowered. You could easily have an ability that has instant KO on target (as an example) if you limit it to non-NM and a fairly low success rate you won't be breaking the game, I suppose what I am trying to say is if they focus entirely on balance then each job should flush out nicely.

3) Basically there are only a few 'roles' in a party: Hate control, support, debuff, damage dealer, tank, healer. The trick is how they get the task done. If you notice there are 4 jobs that require the Sentinel Class leveled and yes you can assume that more than likely means 4 tanks. What would make them different from each other? Easy, Bishop could be an evasion based tank, Defender could be a damage mitigation tank, Paladin can be shield/parry tank, and Templar could be a "take care of myself" tank (ala FFXI Dancer esque; as the Dancer in XIV I would like to be more like the Support Dancer in other FF games) 4 jobs all the same 'role' but they go about it differently, if SE would make this a notion prior to releasing or at the release of these jobs, they can make enemies that each of those tanks work better against.

Okay, another long post, but I hope that clears some things up.


EDIT: Fencer does become Red Mage in that setup, Fencer was not a red mage in previous installments of the Final Fantasy series, it was a class on its own.