Merfolk is my favorite tribe, and after Lorwyn released, I went crazy with different Merfolk decks. I played Fish in Legacy, and I have casual decks built around milling merfolk and an infinite turn combo with Wanderwine Prophets. However, since Modern is now a thing, I figured I'd give it a shot brewing something up.
EDIT: For a slightly more budget and slightly less aggressive list, I figure it's safe to run 4 Dismember instead of PTE. Then, you also don't need UW duals, and can think about dropping Cosi's Trickster (due to no more forced shuffling from PTE) and put in 4 Reejereys, or other Merfolk to taste. Then, you can have a monoblue mana base and be cheaper and more nonbasic-hate resilient. Unfortunately, it'd still contain Aether Vial, but that card is absolutely essential to the deck.
EDIT EDIT: Now that M14 is out (we're going to ignore the fact that I totally missed it), I'm looking at Silence and Tidebinder mage as sideboard options. I'm super glad Silence got reprinted for (predictable) combo and control matchups, and Tidebinder looks like good sideboard tech. Any thoughts on numbers and what to replace?
Staples from ever Legacy Fish deck return, with Cursecatcher, Silvergill, the Lord, Aether Vial, and Leak. I think splashing white is a very solid option, because Wanderwine Hub has no drawback in a Fish deck, and it gives access to PTE, one of the best removal spells ever. I love that there's now Lord of Atlantis 5-8, and Phantasmal Image is actually looking pretty nice. He fits snugly into the 2-charge Vial curve, and can function as another lord, or a Cursecatcher to counter, or copy something useful the opponent has (like a snapcaster or goyf).
The format looks like it's filled with greedy mana-bases, namely ones full of duals and fetches. So, I tossed in Spreading Seas, since I figure it would bone Tron and Jund-style decks. Also, I like Trickster for punishing fetches and tutors, as well as its beneficial interaction with PTE. Shocks seemed like a little too much pain for aggro matchups, so I plan on bringing fastlands instead. Without the shocks, I can't justify running Sejiri Merfolk, which I would love to do, but there's no way I'm including plains with so much UU lord action.
The sideboard is pretty fair, but nowhere near perfect. I've got Kira for anti-burn, and Spell Pierce in cases where I need more answers to non-creature spells. The other Reejerey and SS are in there in case I need to fill that out. RIP and Recall are for more hate against graveyard decks, Affinity, and other stuff I might run into at my LGS. In all likelyhood, this SB is going to change drastically once I see what I'm sleeving up against, but still, I think it needs the most work!
I like the idea of Fishies as an underdog competitor in this format, and I've always loved the tribe, so I'm willing to take any and all advice and suggestions into consideration! Cheers!
The game plan is very straightforward: beat them down with your herrings. See; aggro.
Turn one, your ideal play is Cursecatcher or Vial. Trickster can be nice to throw down immediately, but it doesn't give you as much versatility as the other two. Getting Vial down ASAP is important, and once you do, make a decision based on your hand. Do you have Cursecatchers in waiting that you want to Vial in to counter things? Or do you have other more desirable two-drops you'd like to vial in, like Phantasmal Image or a Lord? Most cases, you can afford to just drop the Catcher and get the Vial to two counters and keep it there. As much as the element of surprise is awesome, we need fish on the field.
Further turns should run pretty easily. You're looking to get an islandwalker lord out, and if they don't already have an island, your enabler is Spreading Seas, which incidentally also usually screws them out of a color. Even without it, you can have consistent, strong beats. After turn two, you should keep mana open for a Leak or PTE, in case something goes unexpectedly.
Aether Vial is absolutely essential to this deck, and the tricks you can pull off with it are nothing short of excellent (the other reason I want to play this is I dread Vial getting banned in Modern, which I think it might). Its ability puts creatures onto the field, as opposed to casting them, and this doesn't use the stack. So this means, Vial-ing something is uncounterable, and can be followed up with instant-speed abilities without giving priority. The big trick is Vial-ing in a Cursecatcher when it has one charge, and immediately sac'ing him to counter a spell, all of which can be done when tapped out. Also, if you vial in Silvergill Adept, you avoid his "reveal or pay three" drawback, since that triggers upon cast, not ETB (even though that's not much of a drawback anyway).
Other tricks include vial-ing in a lord to stave your fishies from burn or for combat trickery, vial-ing in Phantasmal Image for an uncounterable 1 for 1 against legendary creatures*, or just casting one of your creatures at instant speed without having to worry about counters.
* This no longer works due to the weird new Legend rule, but at least you can get your own Kiki-Jiki, right? Ugh.