If the rumors are true and next expansion we are getting both Red Mage and Samurai I seriously doubt they are both going to be dps jobs. One is probably going to end up as a tank. Red Mage will probably be the sword dps.
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I think a lot of people are calling for Samurai as DPS and Yoshi-P even said himself that he thinks that SAM should be DPS. While out of character, Red Mage could be a tank job with defensive spells and a high focus on parries and evasion. If a class that was devoted to sacrificing it's health for more damage (DRK) can be a tank, then so can a caster.
I wish they would expand the cross class concept a bit to allow jobs to draw from more than 2 base classes. Would be interesting to have a lot of the above available to a tank so you've got more viable options to choose from depending on if you're doing dungeon runs, playing solo or fighting bosses. For instance, swift/surecast on the PLD as an option.