I think it would be really bad ass for class fantasy if the Nightborne turned into withered. Now stay with me.
You have a week to go to Suramar and collect anient mana around the zone, if you don't replenish ancient mana within a weeks time, you turn into a withered and lose 10% to all of your damage and healing. (it won't be a lot of ancient mana so it's not a huge chore)
You have a week to do it so it won't be too terrible of an issue, and it always keeps you on that little brink of fear of losing dps at one point if you don't get it done. you will already know about ancient mana since you have to be exalted with the nightfallen to unlock the race to begin with.
It would be cool for class fantasy and I could also see other races having to do similar things but obviously not collecting ancient mana.
I would love to read some more expanding on this.
Edit: Totally forgot about them being cured by the fruit. Thanks to the people that expanded on the idea instead of just flaming.