I just started into Arena a few days ago and loving it. I don't have a lot of gameplay, but one suggestion would be to have story mode. Kinda like the Homelands comic. Like character backstory for first 5 levels. The next 5 levels will be progression to setting up homeland and deck, then the next 5 is journey to the outer worlds and the last 5 is going up against the boss and try to save Dominaria from Xenegos.
Sounds like a fun RPG game too :)
This, but they should just add 5 new "battle challenges" with each new set. The old MTG games had a story mode like this, where each plane would have 5 bot challenges. I believe the best implementation would be for MTGA to provide a sequence of 5 bot challenges for each new set. First, you have to beat the challenges in order using a limited set of cards/decks from the new set. Then, you can re-attempt the challenges in "hard mode" and use any explorer deck.