A mode to even out the playing field for people who haven't played as long as everyone else, and overall just make the game a bit more silly.
Had a problem during the playtest where my friends who just picked up the game and me who had experience had an extreme card gap, since I had leveled + powerful cards and they only had defaults which had a surprisingly noticeable effect on gameplay, a draft mode where everyone gets the same buckets of cards, but can make their decks with 3 unique choices (pick 1/3 cards, lose the rest, rinse and repeat (or a "open X packs, make deck from cards)) would add to the experience in a meaningful way and make it feel less out-of-match RNG reliant, since some decks are just naturally stronger than others.
Since the deck size is 25 this could take a while, hence why I recommended this to a private lobby system where people generally don't care and are just having a good time, but a potential Hearthstone Arena system could also be interesting where you get 3 "lives" with your deck and have to play with it and if you lose all lives you lose it, but if you win you get some arbitrary reward (like a skin for getting 10 wins on a character).
Could also be a good way to have people discover and toy around with new cards.