If I had an Ice Block (I don't) I might consider a second Arcanist, but right now I just can't see what it's good enough to replace. Drakes are too important for card draw and spell power, Water Elementals are too good against Warriors and Hunters, which is like 25-35% of what I'm facing right now, Loatheb's too good against everyone, and everything else that costs 4+ is minion removal which the deck needs. If Warriors and Hunters became less dominant I might put a second one back in instead of a Water Elemental, but right now I'd rather have the ability to freeze my opponent than a big threat. (3+ medium sized threats is harder for people to deal with I find, especially when they're out before flamestrike is castable.)
If you are finding that Feugen and Stalagg aren't doing it for you Azure might be good. It's a really good card in mage, but then I say that as someone who runs Arcane Missiles and Fireballs. Loatheb is an amazing 5 drop, and opponents really hate it when you duplicate him (3 turns of no spells). Loatheb would probably be my top recommendation for a 5 drop in a secret deck. If you end up with 3-4 cheap minions on your board often, a Sea Giant or a Frostwolf Warlord would both be great. (Frostwolf's are maybe not as big as Giants, but they are more reliable to get out earlier so it's a trade off.)