Point form list of what is needed right away (note that many of these can be implemented very easily and quickly)
-improve all event rewards
-card packs now at 180 for 3, and 300 for 5 with bonus rare - change to 180 for 4, and 300 for 7 with bonus rare or better
-PW and card balances, specifically original 5 PW and adjust Baral!
-Duplicate cards! (allow converting dupes for crystals/jewels)
-make epic packs only contain masterpieces (and make mythic only and rare only packs too)
-Fix the RNG (random number generation) this applies to card packs, in game card draw, and matchmaking! (I am quite certain the last one you got has a direct influence on the next one, be it in packs, draw or opponent!)
-deck slots (you said this is coming, but it is needed now - for a temporary quick fix, possibly allow to set cards as favorite)
-purge cheaters
-more communication from devs
Also to consider:
-new event formats such as: Block Format events, Planechase, Commander, sealed/draft events, weighted events (limit rares/mythics, decks of certain point range…)
-more story mode and heroic encounters
-A one day overlap for all things cycling in and out of the vault
-consider allowing decks to have a minimum of 10 cards but a max of 12
-be able to see secondary objectives before selecting PW
-event ranking system needs rethinking (ties, timezones)
-make fabricate animations to and from a single target be all be done at the same time
-The Tier system needs to be relooked at now with 1060+ cards in the game. The cutoff line for the 3 tiers needs to be reestablished with each expansion, or a new tier made (this is fine if it means people suddenly being in a different tier at each expansion)
or possibly take the tier system away from card mastery completely and base it on games and events played with win/loss record considered