Archmage: Have one mage master all the elements (and it would be doable, since the skill trees are much shorter now, according to AH's blogpost)
EDIT: I hope there aren't too many useless achievements like "Finish the tutorial", "Kill your first enemy" or "Gain a level up". Many games do that, and I don't like it, because you didn't actually
achieve anything and those useless achievents just tend to break the atmosphere.
But optional objectives (in LOG1 there was a whole optional level) should definitely be rewarded. And I propably already said this last year, but I'll say it once more: since AH doesn't have to rush with this one because the first game was a big success, I hope they make lots of optional parts to the island for the most adventurous to explore. I LOVED the optional level in LOG1. I actually missed it on my first run, so it added a lot of replayability. (Hopefully that wasn't too offtopic

)
Who needs torches?: Never use any lightsources