last edited by Topical_Island
@naixin In my limited experience with Merfolk, I've found that Walk itself is just great. It's so good that you often win when you draw it, and lose much more often when you don't. This deck goes lethal on turn 4-6 quite often. While that would be impressive in any other format, it makes Merfolk solidly "mid-range" by vintage standards. In my experience, that means that in many games this deck wins or loses by a single turn. Therefore, Walk is one of the best cards in the deck, as I think you've correctly recognized. However, a second, worse Walk seems like diminishing returns to me, precisely because Walk is so good. If you are going to tutor, just get Walk. If you natural draw one of the two, you want it to be Walk. And because Walk so often wins by itself, you rarely need a second. If you did need a second, then that means that the same job could have been done in most cases by an accelerated clock. So in most cases, a Lord would have been as good, and Lords are much better to draw in an opening hand than is Mastery. While we can't say that adding Mastery is strictly worse, I think we can rightly predict that testing would show that it loses more games than it wins if added.
The first card I would add to the list is Mox Pearl, if I had space. It lets you drop damping sphere on T1 against shops at a much greater rate, as well as gets you up to Hurk a turn quicker against Paradoxical. I reckon it improves our post board rates a lot, because it lets us get Priest and Null rod and Hurk and Flux and Damping on the table much quicker. (We are at 2 mana or more 10% more often on T1 by adding Pearl.)
Brainstorm looks nice too. I would have to test on that one. 4 fetches isn't a ton. But I reckon this comes down to using mana efficiently. It feels like this deck will already spend almost all of it's mana every turn?
Lastly, Chart a Course is a new card since the last time this deck was good. It at least needs a look. Part of me doubts it gets in, somehow, just because of the 2 mana cost? But I also wouldn't be surprised to see lists that run it in multiples either. It does not create damage, but lets us toss our extra lands late if we need to in favor of more useful resources. So it's overall effect is likely to slow our clock somewhat, but to give the deck more finish to power through a rain of Swords against Xerox on turns 5-9.