Well, there's a bit of an issue with what you are suggesting however. What you're suggesting is to allow new players (or returning players) to be able to play DRK/AST/MCN if that is why they want to play the game. Great. I have no problem with that. So now how do you do it?
First, we have to remove the "lore" gate of Ishgard, so the trainers for the 3 new jobs have to be placed outside of it (since only the Warrior of Light can enter Ishgard under special conditions). That's not too bad really, though I've not leveled each of the jobs to find out if their own storyline is tied to Ishgard or not. If it is, scrap it and change it so they are instead tied to Uldah(DRK vs PLD), Limsa (MCN), or Gridania (AST).
That really shouldn't be too difficult to do. But, wouldn't existing players complain? "I just spent $40 on this new game and you want ME to level up in an area that I already have spent the past year or so playing in?"
Let's say we brush that off. Who cares what the existing subscriber think anyway? If they're playing, they're likely to keep playing anyway. So now, to point two...
Let's start them at level 1. No need for MSQ. So how do you level? SInce you don't want to do MSQ, I'm guessing sidequests is out of the question too. How do you accomplish this? The dungeons are locked until you go through the story and they open up. So what? Get rid of this too? Should SE auto unlock a dungeon as you fate grind from 1-50 and hit their requisite level requirements? I mean, there's dependencies on these quests, which means there's dependencies on the actual code that allows access to them. No problem, just break every dependency that a quest for Sastash has, for example. Great.
Wait, but Qarn has dependency to Sastasha. No big deal, break that too.
But, Hauke has dependency on Qarn too. Oh crap, gotta break those too. And so on, and so on.
And now what if you have a new player who actually wants to play the MSQ and go through these stories? Now comes the difficult part, because you have to make sure that while you're breaking these dependencies for the non-story people, that you have to make sure they also work for those that DO want to do stories.
Basically, this is an endless cycle of broken quests and bugs that is waiting to happen.