Leveling your first character is fun. You can do either quests or FATEs. However, after leveling your subclasses to reinforce your main class quests become scarce, and by the time you want to level a 2nd class to 50, there are almost no quests at all, leaving one to grind FATEs (which has been more than adequately complained about in these forums).

I therefore suggest that quests respawn after a given amount of time, maybe a week or so (but I'll leave that up to SE).

Some games (FFXI included) had repeatable quests, but those can get to be as much of a grind as FATEs. Still, having some that you can repeat three or four times may not be bad.