Normally, the 15 cards would just be cards that are boarded in games 2 and 3 to improve certain matchups where possible or needed. However, in this deck the wishboard serves a secondary function as a place where cards can be fetched with Living Wish to adhere to the many situations that will occur while playing this game. You could forgo the whole "outside the game means your sideboard" rule enforced in tournaments if you wish, but I made this with the aforementioned rule in mind. For convenience's sake, I separated the cards intended to be boarded in, and the cards where at least one copy is intended to be kept in the sideboard.
Wishboard
Boseiju, Who Shelters All Cavern of Souls: This makes living wish much more of a threat than it normally would be vs control decks. If these cards were not present in the wishboard, a control player would probably just counter the primeval titan that comes from the wishboard. By having the ability to wish for these cards, the control player now has to respect living wish and counter them, paving the way for cards like Green Sun's Zenith or actual primeval titans. There are two copies so that a copy of each land can be boarded in to help punch through attempts at countering the Primeval Titans, which makes Crop Rotation also a serious threat that needs to be countered, lest the control player likely loses the game.
Reclamation Sage: Cheap and versatile tool for destroying problematic artifacts/enchantments you may face off against, such as Blood Moon or Pithing Needle. A copy can be boarded in to give Green Sun's Zenith additional utility as a potential tutor for artifact destruction.
Riftsweeper: This is probably one of the most obscure and least-commonly wished for targets. This deck is incredibly resilient to attempts at exiling Primeval Titans from deck, but that same fate happening to Emrakul or something can prove to be deadly. Riftsweeper provides an out to these situations. Again, a copy of this card can be boarded in so that you have 8 ways to search for this guy (green sun's zenith and living wish).
Sideboard
Krosan Grip: The best artifact and enchantment removal there is. If you don't face much stack-based interaction, then Nature's Claim is probably going to be better since winning with this deck isn't as contiguous on the life total as many other decks are. These cards tend to be boarded in most often in place of All is Dust, either because of lack of permanents to remove (from control decks probably) or from decks largely operating off of colorless cards (artifact-based, or that one guy playing Colorless Eldrazi Stompy in casual)
Bojuka Bog: Graveyard hate for those wanting to play unfair, this one is intended to be boarded in as a way to gain instant-speed graveyard hate with Crop Rotation, or to search up with Primeval Titan to secure a game. Keep in mind Vesuva can copy Bojuka Bog triggers if need be.
Kozilek, the Great Distortion: Honestly this card should probably be replaced, I moved Karakas to the mainboard and don't know what to replace it with. I could see this card being slightly better vs combo or something, may be a wishboard target if you already used Primeval Titan.