Player retention is a big challenge as, people stays only if there is new content and if they feel themselves progressing.
Maybe a story mode and weekly/season events could help to keep people engaged in the game longer as it creates a variation of gameplay (and not only pvp battles 100% of the time). However, that would take a lot of time and efforts to build these elements.
To increase player adoption, it could be interesting to offer a "free to play" mode and a good Android/iOS app.
One way is to integrate an NFTy arcade like deck renting system directly in the game's interface (the cards could be provided by anyone, SPL, 3rt party app or players).
Integrating this solution to the game's interface will allow more people to really experience the game and hopefully keep them longer in the game.
Actually, I would go even further by allowing new players to rent decks even without the summoner's book, however in this case the player's share of battle rewards would go directly to SPL team (no rewards without summoner's book).
Edit: corrected typo and syntax