Bliz made Wrathion be an “advisor” to Anduin in order to make a parallel to how Onyxia had been an “advisor” to Varian. That is, they want players to keep worrying that Wrathion might actually have something sinister up his sleeve, and he might end up doing something to really make you believe that he’s evil-- but it’s clear to me that Wrathion is going to end up a ‘good guy’ in the end.
Anduin is going to be temporarily influenced by the shadows (and temporarily become a “bad guy”), which is the main reason that Bliz was so adamant about keeping him a priest instead of making him a paladin-- and most likely it’s going to seem like Wrathion had made him turn to the shadows or something (like how Onyxia had “evilly influenced Varian”), but it won’t actually end up being something sinister on part of Wrathion. In the end they’ll both triumph as heroes and fight together side by side. But there is likely to be some deceitful thing on part of Wrathion that will make you suspect or dislike him again.
That said, really trying to stay away from the lore on the whole because all it does is make me upset. If you ever want to try and figure out what bliz is intending to do with their stories, just think of the most misogynistic possible route and go from there. Never fails.