1. A re-balance. Cards rotated into the classic pool, Naxx is free, BRM is half off, cards buffed, cards nerfed.
2. Arena re-balance. Tacked on to #1, Blizzard needs to just make separate Arena rarities. Keeper of Uldaman is the best example I can think of, it's an adventure card, meaning the rarity doesn't really mean shit, so it has NO business being a common.
3. Swifter response to patching, and a public test server. On some level I can appreciate Blizzard taking a hands off approach for a while on cards that may seem overpowered but come the fuck on. Mad Scientist and Dr. Boom have been out for a year, Mysterious Challenger and Paladin in general is running rampant and playing the most OP minion on curve destroys nearly all tactics. A Public test server would be good to actually get people playing these cards before sending them out into the wild. Note this doesn't mean cards should be hamstrung when nerfed, but this IS Blizzard, so that'll never happen.
4. Better rewards, arena and constructed. Self explanatory, even with chests you don't have a lot of incentive to grind up ladder and I'm sure we've all gotten to 10+ wins arena to be handed 200 dust and some coins.
5. Tavern Brawls. Stop repeating, or if you are going to repeat, stop repeating shitty ones. Make deck building brawls have every single card available for use rather you own them or not, this is supposed to be the fun mode right? Also, for the love of God, just ban certain cards for certain brawls so they can be playable beyond one strategy.
6. Mechanic reworks. Specifically thinking of deathrattle, silence, charge and joust. First three for being too damn good and the third for being too bad. I am unsure how to go about this, but charge has been a problem a lot of nerfs (Leeroy, Warsong, Charge + Giants) and existing cards like Force of Nature. Deathrattle promotes vomiting high value minions like Shredder and Haunted Creeper on to the board that are obscenely hard to remove and trade well, leaving few options for swift and easy removal. Less of a problem with the Undertaker nerf though. Silence is, well...I don't think they ever intended silence to be used to drop a 2/1 that will negate any massive taunt. Joust is bad. You play a Joust card to get a slightly better version of an existing card (Gadgetzan Jouster -> Zombie Chow, Master Jouster -> Sunwalker) on success but a strictly worse card on failure. Healing Wave, Tuskarr Jouster and King's Elekk are the only joust cards I have seen see any sort of play at all, and even then they are hardly worth it. Joust needs to either have the person who initiated the Joust win on ties, or simply do the joust again.
Shout out to secrets because I always thought they were a stupid mechanic but Mad Scientist and Mysterious Challenger made them so much worse
7. Nozdormu. Either fix this piece of shit or remove him from the game. As is, he is an allowed exploit.
8. Animations. Tacked onto Nozdormu, some animations take WAY too long. You could make the argument this could have held back Patron, but the old Patron is long gone.
9. Death Knight. Because Death Knights are rad, and there's nothing that would fuck the meta up into a state never before seen then throwing in a new challenger.
10. Shaman. Do I even need to explain
11. Ladder. Its bad. A new player at Rank 20 can run into a Control Warrior farming golden portrait, who they cannot even be expected to beat.
12. Deck slots. This meme was never funny. I just want more deck slots.