Altering quest is a lot to work on though, to cut the fat of some quest and clean up the narrative. it would take a lot of work to balance it while also making sure they don't leave out anything important. The best way to actually "fix" the issue is adding in shortcuts based on the completed journal list.

So to skip the quest requirements for 2.1-2.55 content you would need to complete "The Ultimate Weapon" They would then add an NPC you talk to that would confirm if you wish to skip the quest and be able to unlock "Coming to Ishgard", that while one would be able to go back to it later this bypass would just allow the user to start the content from here.

A Second potential bypass can also occur with that quest unlocked, "Coming to Ishgard" you could bypass it but with an added bonus that I feel would make sense given that 4.0 zones will most likely start at lv 60, allow the player to skip through 3.0-3.3 starting from "Promises Kept" and giving said players all aether winds and one job of their choice that is 50+ to lv 60. However this means they would still need to get the aerethytes in the area but the zones woudl be as if they had already completed 3.0.

But see that's why it's tricky, I do believe the best way to do it is to allow new players to start with the newest content as soon as possible and leave the door open to let them go through the old content once they've had their fill.