While I personally think your idea has some merit, I can see at least 2 possible problems right off the bat:

1. the crowd that LIKES predictable content, that prefers that everyone "know the fights" ahead of time, would rebel against anything that makes it take longer for people to learn the fights, even if your idea WOULD make things more interesting. I can't speak for how big that crowd is in relation to the player base as a whole, but given how often I tend to run into people with this mindset, I'd expect more people to rebel against this idea than embrace it.
2. regarding the job-based shortcut idea, the devs would have to implement a way to make EVERY class/job useful in in SOME dungeon somewhere, or someone somewhere would cry fowl when their main job isn't given a way to shine (some might even complain if their job isn't given a way to be useful in EVERY dungeon). As an example, I main NIN, and the only way I can think of to implement a purely NIN-based shortcut would be to create some kind of interaction point that I'd need to use stealth to reach (anything based on my elemental Ninjutsu could also be done by BLM); not only would that be tedious given the movement speed restrictions, but if I screw the shortcut up, I'm now cut off from the group and will likely get killed, wasting even more time while I respawn and rejoin the rest of the group to either try again or proceed normally. Other shortcut ideas could also have similar problems if they aren't automatic successes, and given how some jobs operate, not all of them CAN be.

Don't get me wrong, I'd be all for this, if for no other reason than to see what the devs come up with to make things more interesting...but I seriously doubt the majority of the player base would support something like this.