To set the stage for what I am about to ask for currently I have two characters, one DoW 50 character but primarily for DoL/DoH maxing. A second character with also DoW 50 that is going to be used primarily for all DoW/DoM maxing.
Now I enjoy the main story quests, they are a more fun way of breaking up and using to level a DoW/DoM class (primary and/or alt class on single character) rather than just grinding fates/dungeons and leves. When I level alt classes on preexisting characters I end up in cities and locations with zero quests to do at all right now since did them all once before on that character so all I am left with is grinding what I mentioned above which puts me off maxing other level 50 classes.
I would like the opportunity to replay the main story line quests on each DoW/DoM class I want to level on a single character. Given any new class starts at level 1 again this means there is no major advantage will be gained enough to invalidate the progression or completion of main story content. This game allows you to repeat leves, repeat dungeons, repeat fates and repeat hunts on alt classes on the same character but does not allow for the repeating of quests unless make entire new character instead of making a new class on a pre-existing character.
I am not saying however a new alt class should be forced to do main storyline again but instead giving an option to replay quests already unlocked. the only major issue is quest rewards could be an issue for the end period of main storyline since gives high level materia but if push comes to shove the quest rewards can be turned off by SE for the replaying of quests previous completed with exception of Exp (and Gil maybe). I know would not be an easy thing to implement but also do not believe would be an extremely hard thing to do either, it would greatly improve and make more fun the leveling process for alt classes on the same character.
My idea is just a rough idea and I am sure SE could refine or improve it but overall I do not think would be bad idea and would make playing alt classes much more enjoyable especially given there are so many DoW/DoM classes available to level after got first one to 50 on top of being potential incentive to help more people be willing to grind more tanks and healers to 50 if not put off by boring themselves to death grinding fates, leves and dungeons only each time start new class on same character.
It does not even have to be limited to main story quests but also any quest once completed can be repeated if wanted, allowing for the option to repeat any quest restricted only by level requirements just like previously was when did it first time and perhaps turning off the item reward and only giving the same Exp and Gil reward that originally had when did first time. Altering the journal so has a tick box on each previously completed quest, when ticked a box for a quest it reinitialize that specific quest which can then repeat it. As said my idea is basic and probably lacking in refinement but the core principle is the allowance of repeating previous completed quests in general, the restrictions SE can decide and how go about it they decide as long as it gives Exp as before to help leveling an alt class on top of simply doing for fun because enjoyed it previously.