I don't see the point of doing the story more than once. People who roll alts in other games just skip the quest text anyway since they've already done it. All you're asking for is to make an already easy way to level up even easier. Your secondary classes get a 50% xp boost for random kills, levequests and FATEs, and even more from dungeons. And this doesn't include the daily roulette bonuses and daily beastmen quests in the 40s.