Assuming fatigue won't be going away anytime soon....

1) Raise the threshold values whenever SP gain is increased. If you're going to rebalance SP so that players rake it in faster, then dangit raise the threshold so that we don't hit it faster. I know it's obvious, but it doesn't appear to actually be happening.

2) Make the classes more individualistic so there is actually a good reason to level them simultaneously past 20. Why do so many classes need a Bind effect, anyway? Or a "hit all enemies in a line" attack? Instead of cloning abilities across classes, separate them so that people will actually *want* to cross-class for them.

And a couple more related specifically to fatigue accumulation:

3) Turn off fatigue generation when in a party. If you're in a party, you don't generate fatigue. At all. Oh hey look, suddenly everyone wants to party.

4) Bonus SP/XP from levequests and Guardian's Favor does not add to fatigue. Only the enemy's natural SP/XP values add to the threshold counter. Any bonus you get on top of that for being on a levequest, or having Guardian's Favor active, is just that--extra free SP/XP that doesn't incur a penalty.