I like the idea of the Magian Trials... a long, challenging adventure. I like the idea of a quest going on for more than 20 hours of gameplay... as long as:

1. There are a lot of cutscenes, story, and dialogue. There is an overarching storyline to keep players interested, and an amazing ending and stuff. Like a huge side-story.
2. Explain why I would have to travel from this place to a gigantic mountaintop just to kill the enemies there.
3. If you're going for the grind, shake it up at least. Not 400 of these mobs... how about 50 of this, then get a linkpearl call to go kill 50 of that after walking to another interesting place. Break up the grind with travelling to areas that I normally wouldn't, or add new areas to compliment it.
4. If you're going to implement killing bosses, meld them in the storyline. Add new bosses, too, or stronger versions of older bosses in their own instances through the storyline.