So this is, largely speaking, the final iteration of my Merfolk Tribal deck for EDH.
As with some of my others deck lists, I like to explain some of my choices regarding certain cards and/or themes within the deck. Furthermore, I will also give some advice on other cards which could be added or removed in order to change the direction of the deck - or simply make it more budget friendly.
To begin, there is a small Voltron package within this deck as it can be quite easy to turn Kumena, Tyrant of Orazca from a 2/4 into a much larger creature. In addition, Kumena has in-built evasion to chip through commander damage or, failing that; Islandwalk from Master of the Pearl Trident and Lord of Atlantis; or even unblockable from Herald of Secret Streams. Ultimately, combat damage is your win con (I'll touch on this more later) and therefore having evasion and a big commander will push you towards that goal.
However, the primary function of this deck is to value out your opponents. Cards like Merrow Commerce, Quest for Renewal, Seedborn Muse and Murkfiend Liege all untap - at the very least - your Merfolk to be able to draw cards or pump your team on opponent's turns using Kumena's activated abilities.
In a similar vein, Sword of Feast and Famine untaps your lands allowing you to vomit out even more of a board state after combat or simply hold up mana for interaction during other's turns - the same can be said for Cryptolith Rite, Song of Freyalise and Jianng Yangu, Wildcrafter in conjunction with an untap effect.
I am no longer running as much of my instant-speed effects, such as Leyline of Anticipation and Alchemist's Refuge. This is simply down to cutting some of the less impactful cards in my deck, as well as just generally having more instants anyway to be able to interact regardless of flash speed. Furthermore, having too many colourless producing lands or dead enchantments can really hinder the speed of the deck.
To round out the value, there is a suite of creatures within this deck that have either valuable ETB, activated or triggered abilities. It would take me all day to list and explain all of them, but to name a few: Merfolk Trickster grants unique interaction against opponents creatures by essentially neutralising them for a turn; Enclave Cryptologist, Benthic Biomancer and Thassa's Oracle are draw/filter engines on 1 creature, synergising well with the proliferate mechanic; and Kopala, Warden of Waves gives pseudo protection to your Merfolk. In addition to these, I run a pair of amazing clone creatures which help grant additional value to your creature suite. Phantasmal Image can copy any creature for only 2 mana (yes, including your opponents' creatures) and, despite the static ability, the card is still insanely good. The new inclusion along with the Image is Glasspool Mimic
, which is not the best clone spell in the world - though 3 mana to clone one of your creatures is still not a bad rate - but it is primarily included due to the ability to also play the creature or land side. Having a choice of spell or land thanks to the new modal cards from Zendikar Rising is underrated, as said by many pro players and content creators, mainly due to increasing the rate at which the card will be relevant in the game, not to mention that most can be played in place of a land from your deck as well.
Honestly, whilst I will happily explain any inclusion in this deck, they are, ultimately, self-explanatory for the most part.
In terms of potential inclusions, however, there are a multitude of ways to take the deck.
For example, Isochron Scepter with Dramatic Reversal or Benefactor's Draught for value and a potential infinite mana combo. Similarly, Incubation Druid with these or Vigien Grafmage/Freed from the Real also makes infinite mana (don't forget Cryptolith Rite and similar effects turn your Merfolk into mana dorks). Chord of Calling, Sylvan Tutor, Worldly Tutor, Muddle the Mixture and Collected Company are all valuable and powerful tutors to gas up your deck with.
If you wish to make the deck more budget friendly, then simply cut some of the more expensive cards and run suitable replacements. More merfolk, for instance, can't hurt. Some of the Changeling/Shapeshifter creatures can help fill out some gaps (see, maybe, Adaptive Automaton for an additional lord effect). Alternatively, lean harder into the +1/+1 counter theme within the deck. Recommendations here might be cards such as: Vorel of the Hull Clade for doubling counters (Doubling Season if you can afford it), Prime Speaker Zeganna to draw lots of cards, Combine Guildmage to move counters around, or even Simic Ascendancy for a straight up win condition.
To sum up, I have taken my decklist in a very particular direction and customised it to my playstyle. But, there are many different things to do with Merfolk as a tribe in EDH. It can be budget or less so, regardless pick a direction a run with it.
(Note: I do not own a copy of Tropical Island. I use a proxy copy, which my friends allow me to play because fixing in enemy colour decks is not very good).