The start could be like a short version of the combat part, and no 50 hour preparation part.But then it'd more go into different segments where you need a few specific classes per path to know their job, and the others can just mindlessly zerg, which seems to be the main idea behind Mid-core. So for example, in 1 path you need an experienced bard and dragoon, and the rest of the party can just be any class.
The reason people won't farm 1 path, is because different paths drop gear for different classes

Maybe there is a certain level progression in gear, but thats not always a bad thing, because if you don't offer gear, but something like mats/mounts/pets, people will stop doing it the moment they get their item. Plus not everyone can even use the rewards, if you were to offer special crafting mats, cause not everyone does crafting or gathering.