Deep Breath. Let's dive into Rivals Merfolk.
What should the main strategy be with a Merfolk Deck?
Before Rivals, it seemed like Merfolk had the potential the go wide with token generation viz-a-viz Deeproot Waters and big with a couple cards like Verdurous Gearhulk. Putting Tokens on River Sneak gave you a pretty powerful unblockable creature. I created a Sultai Merfolk deck with Winding Constrictor that could get pretty crazy. Two questions are whether there's room in this deck for support cards like Rishkar, Peema Renegade and whether counters should even be a major theme in this deck.
In playtesting this deck it seems like there are a couple crazy interactions possible with the New Merfolk we have. Obviously Kumena, Tyrant of Orazca rewards you for going wide with Merfolk, and there's some cards that will give you five plus Merfolk early on. Playing Jungleborn Pioneer with a Deeproot Waters out nets you three fish. Having a Deeproot Elite out also lets you put down three counters. It seems like Merfolk wants to go wide early on and drown your opponent in card advantage with Silvergill Adept and the Merfolk explorers in the early game, and Kumena in the late game.
Counters may be important because it lets you beef up your fish without relying on the rather vulnerable Merfolk Mistbinder. Mistbinder is the lord the deck wanted, but it dies to every piece of removal in the game. I see it as a good in the early game, potentially turning Kumena's Speaker into a 3/3 on turn 2. Actually, the giant target that Mistbinder presents might serve to soak up your opponent's removal and allow you to play other Merfolk.
This deck wants to play Deeproot Waters as early as possible. Just Kidding, see update below The explore cards in here let you dig for it a little bit, and can sort your lands. Silvergill Adept draws you an extra card. Against control, you want to hold on to Deeproot Waters until you can play it with a Spell Pierce backing it up. After Deeproot Waters resolves, it's pretty smooth swimming against control. They can counter as many fish as they want, and you still get a 1/1 body.
Update: Most of the Merfolk decks I've seen are not playing Deeproot Waters. Playing on turn 3 or 4 basically means you're skipping your turn 3. But given how the meta is shaping up, with control decks with access to sweepers like Bontu's reckoning and fumigate making up about 20 percent of the meta, I think Deeproot Waters is going to be crucial to this deck. If you're not playing 3 Waters, then you really got to play at least 3 Spell Pierce in its place.
Update 2: I actually took out Deeproot Waters. I'm trying to make it work, but against, say, Big Bad Red, if they're on the play they could be swinging for 8 on turn 3 if they have Bomat Courier, kari-zev, skyship pirate and Ahn-Crop Crasher out. It's possible to play Deeproot Waters on turn 3 and also play a body if you can play a card like Enter the Unknown which lets you play an extra land. But that puts too many spells in the main.
A major problem with Merfolk in Ixalan Standard was the potential of getting swept against control. Blossoming Defense doesn't do anything for you against Settle the Wreckage or Fumigate. I'm including 3 Spell Pierce in the Main deck to use against Control, but Deeproot Waters also helps you recover from a sweep. Update: not doing that anymore.
One question about this deck is the mana base. Should you play Attune with Aether and Aether Hub or not? I'm holding out to see if there will be some kind of energy ban before buying a full play set of those cards. What about Hashep Oasis? Update: I ended up putting two Hashep oasis in there, but I might run 4.
Another question is: Metallic Mimic? Like Mistbinder, it's a giant target on a vulnerable body. The card is almost dead as soon as it comes into play. But it soaks up your opponent's removal. This deck just has too many 2 drops. But maybe 2 Deeproot Elite and 2 Mimics might be the way to go. I'm playing 3 copies of a lot of the cards so that I can play as many different merfolk as possible. Update: I decided to cut Metallic Mimic all together in favor of Deeproot Elite. Deeproot Elite lets you pick the target for that +1/+1 counter, allowing you to beef up a specific fish, be it an unblockable fish like River Sneak or a target that's vulnerable to Abrade like Merfolk Mistbinder.
Explanation of sideboard: We have Spell Pierce to play against Sweepers. You might be able to catch a Heart of Kiran with one too. Naturalize is in there too. Post-board I think this deck can stand up pretty well against Mardu Vehicles. We have, of course, Nissa, Steward of Elements which can help put a ton of bodies into play. We have some cards to play against cards like The Scarab God: Entrancing Melody, Essence Scatter, and Kefnet's Last Word.
UPDATE: This deck has been dying pretty quickly against Mono Red. I was thinking of ways to speed up the deck. For example, you really don't want to just play Deeproot Waters on turn 3 or 4 against red after they played Ahn-Crop Crasher on turn 3 or Hazoret the Fervent on turn 4. You want to play a body. But by putting Enter the Unknown in, you can play an extra land on turn 2 or 3, allowing you to potentially play Deeproot Waters AND a body on turn 3 or 4. I also put in a copy of Life Goes On which can buy us a turn or two. Aethersphere Harvester may be an important piece. It's a solid turn 3 play, can buy us some life, and can block Heart of Kiran and Glorybringer. It's also good against Ahn-Crop Crasher because it can't be targeted by its effect, and even if a creature has been targeted and can't block, it can still CREW our harvester.
Post-ban update: taking out Attune and AetherHub. Doesn't make a big difference for the deck. Adding a couple lands.
