Being Legendary is neat, and that will allow decks that care about Legendary matters access to a tutorable board clear
From there, its a balancing act between including enough flying and jumpable attackers, having enough repeatable sources of Constellation that you can use to throw enemies and allies alike into the air, packing enough removal (and recursion for that removal) so you can keep the skies clear, and making sure theres enough engine pieces to keep the deck running smoothly
While they might not be as pricey as the new Umbreon from Prismatic Evolutions , they still hold some value
Reckoners Bargain is a newcomer on the block, and while many will instantly favor Deadly Dispute for the Treasure, theres a bunch of decks where Id be happy to take Bargain first
As a fine card on its own and one that's incredibly strong with other Humans on the battlefield, this is a great example of what you're looking to play in these kinds of decks
END STEP Magic changes constantly, but the old reliable merfolk deck is much the same as it ever was