The most logical choice (for me) is Samurai tank, Red Mage DPS and Dancer healer. I do not enter on Dancer disscuss, because most people agree that healer fit it better (or healer/support).

Asuming that Red Mage and Samurai will come in the same expasion (which is possible not happend), with the current configuration of the gear, fending type gear for tanks, are not sutiable for Red Mage for tanking. Obvius reasons. Make the Red Mage as tank means they will get Fending gear, even if the new expasion drop this armor configuration, the old gear MUST suit the new classes to. So, a Red mage wiht a heavy plate gear won't look very... good. Red mage uses clothes, or as much, light gear. So, red mage, as magic class (they uses sword, but they are still a magic class) will use probably Casting gear, like the other mages. While Samurai could get a heavy fending gear without any problem. It is not good idea to give this news jobs exclusive gear, as NIN, MONK and DRG. It is very stupid and only will make the armoury chest fill quick, and also a more longer gear grinding if you have all jobs unlocked.

More reason. As Red Mage is a caster for obvius reasons, the Samurai relies on parry/evasion is some way, which are most useful for a tank class than melee. (Tell me how many time the DRG uses Keen Flurry compared to the Dark Dance or Raw intuition of tanks). About "there are too much tanks with slashing weapons or two handed weapons". Let's think. It really matter what kind of weapons they have? Also, putting a Red Mage as tank, is not a sword too? Even if it is a Rapier, which can be Piercing. It is still not enough for a reason.

Let's see the other possibility. Both Red Mage and Samurai as DPS. Then. What about players who play tanks? They do not deserve a new class?

Let's make clear. Yoshi-P didn't confirm NEVER Samurai will be a DPS. He said his opinion is the Samurai a DPS class, and in case of being tank, probably name it Shogun. But that's doesn't mean he will do it. Of course, the possiblity is there, ans the same for tank, (Samurai was considered a tank for Heavensward, but they decide put Dark Knight instead). So, can be both of them. Only time will answer this. I hope soon we will know the new jobs, maybe some time after the teaser trailer that is supposed to show in the Fan Fest.