A new skill! 20 sec job change! Idea.

[kyuven]

Then it can be simply edited changed to be balanced out. Not sure why your saying the whole party can change to DPS when I put

There's nothing simple about balancing anything.
The mere idea of what you're asking would basically break the game.
You'd have twitch gamers exploiting the hell out of this, as even with a 5 minute cooldown you'd basically have ALL OF THE CLASSES DPSing down the same boss. You're basically giving every class a second set of their unique cooldowns and taking away a LOT of the challenge from the game. Every single raid team would be: BRD/NIN, MNK/DRG, WAR/PLD, WAR/PLD, SMN/BLM, WHM/SCH, WHM/SCH.
And in order to add challenge they'd need to completely redo the way the game's encounters work. Oh, and if you want to add new classes, you need to add new pairs for them! Once heavensward hits, there will be an odd number of all the roles. How do you handle that?
And even if they DID add it, the testing team would have their heads on a platter because they'd have to double the amount of testing on every fight. Maybe even triple or quadruple.
They purposefully removed the ability to switch jobs in dungeons. They ain't putting it back.

I already came up with a suggestion on this, once you get a Role say, PLD to LV50 you could switch to the WAR Role even if you don't have it unlocked.

Could implant something that allows you to be that role for a few mins to set up your hotbar and get used to the skills.

What's the point of leveling more than one of each role, then?