I run kind of a bant version of the deck and my mana base is 4x wanderwine hub, 4x breeding pool, 4x mutavault, 1x cavern of souls, and 7x islands. I agree with some of the above comments in that good counter magic is hard to come by nowadays and even the control decks play a bunch of 1 and 2 of a bunch of different counters so 2+ caverns is pretty unnecessary until you start playing legacy Merfolk then I’d recommend 4 for sure. I also run 4x CoCo mainboard and it’s actually fun in game 1 when you stick on on your opponents end step. I’m also using the green sources for kumenas speaker to kind of speed things up early. I use the white splash for hate cards out of the sideboard. Currently I have 2 Rest In Peace but in my old meta I was running 2 stony silence which I get is a complete nonbo with my vial but so long as I had a little bit of a board state I was ok and could usually back up the lack of vial use with a CoCo. I’ve debated on rec sage but I believe in those matchups hurkyls recall will still suffice. Even if you’re bouncing their vial that’s on 2 or 3 it resets it and buys you time.
Here’s a link to my list for those that would like to take a look. https://www.mtgsalvation.com/decks/10186-merfolk
A Viking world would be sweet. But I’d also like to see some of the planes walkers go back to their home plane and see what’s going on there. Origins kinda did it but I’d like to see each individual plane in all its glory from the feature planeswalkers perspective kinda like we saw kaladesh from Chandra’s perspective.
Here’s a link to my list for those that would like to take a look.
https://www.mtgsalvation.com/decks/10186-merfolk