I'd suggest Gitaxian Probe. It's basically 2 life to give your Fiend +3 and also a card draw. In that same line, Manamorphose is a must, as it's a free spell and free draw. Basically, any card that can count as a free spell even at the risk of having to sacrifice, like Lava Dart and Fireblast, or any cheap cantrip card to help with keeping your hand filled. Also, Wee Dragonauts.
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What kind of deck do you usually play? Or for that matter, what kind of deck do you want to play? I don't really play standard much, except occasionally MTGO. But from what I've heard it seems U/W control is really strong. Humans is a great tribe in Innistrad. G/R Werewolves is pretty weak, but Dark Ascension looks like it may give them a big boost.
This deck needs Hypno Toad. No check that, this GAME needs Hypno Toad. Get on it Wizards!
See: Goatnapper.
Grim Discovery is great for landfall. Together with Walking Atlus (or Sakura-Tribe Scout), you can drop a TE, sac it and search for a basic land, cast Grim Discovery to get the TE back, play the land with Walking Atlus, sac it and search for another basic land, netting you 4 landfall triggers in one turn.
Elven Warhounds
Correct me if I'm wrong, but I don't think tokens ever hit the graveyard.
Crucible of Worlds and Goblin Razerunners. When you reach more than 5 mountains, sac them to the razerunners and play them again from your graveyard with Crucible. Also, I'd say get rid of Ember Hauler for more 1 red mana 1/1 goblin weenies with haste, simply for the sake of attaching Explorer's Scope and getting the lands out faster.
For being Merfolk aggro it doesn't seem all that aggressive. It seems more like an attempt at control (numerous counter spells, Lullmage Mentor, Acquisitor, etc). If you want to focus more on aggro, then reduce the counterspells a bit, take out the Mentor and the Trickster (and probably the Cryptologist too). Focus more on the Leviathan and islandwalk. Quicksilver Fountain, Streambed Aquitects, and Lord of Atlantis would be great. Also, consider Merrow Commerce, it's just an all-around awesome card for merfolk. Merfolk Wayfinder could also help to get the Leviathan out faster. If you want to focus more on control, then I'd say remove the Leviathans entirely.