Archmage Arius is rarely seen because he is so scarce (limited supply) and because most of the players who do have copies of him tend to have larger collections (with more summoner options) and they play at higher league levels (where he isn't as good).
I definitely think he should NOT be improved further, especially because I think he is overpowered at the lower leagues. Right now, most people tend to just focus on card balance at the highest league instead of considering the impact on all league levels. I've seen Arius used to frightening effect in Novice/Bronze tournaments and there isn't much you can do to counter him at those levels. Boosting him further will just make it worse. Right now, it's just that most Arius owners don't participate in very many Novice/Bronze tournaments.