This is my first attempt at building a functioning Merfolk deck. I will be updating this as I playtest it, and hopefully when Rivals comes out it will get a redo too.
Tested Strategies
The main concept of the deck is to build up counters on all creatures before playing Herald of Secret Streams to suddenly hit for lethal. The combination of Rishkar, Peema Renegade, Metallic Mimic, Merfolk Branchwalker, Verdurous Gearhulk, Vineshaper Mystic, and River Heralds' Boon means there are more than enough counters to go around. Even Kumena's Speaker, a simple one drop, can easily come in mid-game as a 3/3, and then be cheaply bulked up as moves progress.
So far, what seems to be useful about this is its flexibility. Counters can be spread across numerous creatures, which can be quickly created by Deeproot Waters, to be able to compete with cheap and light aggro decks, or stacked up on one to form one unstoppable beast when against dinos or other heavy hitting creatures. Verdurous Gearhulk is great for this, and it can be bought in relatively easily with Rishkar, Peema Renegade. The other benefits of the counters is that key creatures that would be in range of Lightning Strike and Abrade can be bulked up (looking at you, Herald of Secret Streams).
Chart a Course should only be played to get the two cards. The draw backs of the cheap cost to play most this deck is that you need to play a lot of cards to compete with decks with high cost, high hitting creatures, so the card advantage is a must. This is also why Shapers' Sanctuary is in.
Untested Strategies
The sideboard has yet to be finished and tested. The deck can hopefully respond to a control deck by bringing these in. Deeproot Champion can be slowly bulked up whilst playing noncreature spells, so we are rewarded for what we want to do. The additional Kopala, Warden of Waves increases its draw likelyhood, and this works really well with Spell Pierce to protect Merfolk - The Disallow can be used to protect Verdurous Gearhulk.
I may look to make space for a third Deeproot Waters to give greater hand consistency, as well as potentially stacking. At this point, the extra card is sitting somewhere between main and sideboard, but more testing will clear this up.
Finally, Nissa, Steward of Elements could be useful - she can be bought in with plenty of counters on her with Rishkar, Peema Renegade's ability. Currently, however, Verdurous Gearhulk is doing a pretty good job of ending games, so she may not be needed. Makes the deck cheaper by a good amount if she isn't in it too.
Splash Black?
I have toyed with the idea of splashing black in this, mainly to bring in Winding Constrictor as well as the plethora of removal spells, such as Walk the Plank and Fatal Push. Currently, however, this may not be needed. Firstly, Winding Constrictor would either have to compete with Metallic Mimic for space in the deck or for the two drop. Mimic has the benefits of all the Merfolk in the deck of greater protection through Kopala and counters, which the snake doesn't. Yet the attraction of increasing counter numbers is real, so this may come in at some point.
Secondly, Herald of Secret Streams reduces the aggressive benefits of removal. When this comes in the whole dynamic of the board changes, as suddenly it doesn't matter if there is a wall of creatures and tokens - they can be ignored. The defensive benefits do exist, but again, the flexibility of the counters means that it isn't necessary to have to remove creatures- just bulk up in the right place. I don't think the benefits of the removal spells would balance the reduction in draw consistency for the main theme of the deck.
Thirdly, the destabilisation that splashing causes to the mana base, as well as the cost for the dual lands, makes it less attractive.
All in all, more playtesting is needed. I'll keep this updated, and any suggestions would be greatly appreciated.
