General Arena Suggestions
This is going to be a long post of the suggestions to Arena. I want to put it somewhere in case they actually want to do something about it. I've been playing paper for quite a while and have played Arena since Closed Beta. I'm doing this strictly from the Arena perspective.
Deckbuilding - Improve the keywords that you can search. It would be nice to search for the Colorless mana symbol/colorless cards for instance. Things that generate X type mana. Stuff that don't technically have the keywords but should (like Mill before it became one). Going with this, allow players to add keywords that you can approve that are searchable (removal, shock, tutor, bear, vanilla, or other popular shortcut terms we use for effects/types). This might need to be done for your Oracle but it's possible and can be community driven.
Gameplay - Before even playing, if you haven't gone first last game and your opponent did and vice versa, you/they should go first, no check on that when that happens. Get rid of your shuffling algorithm for a truly random one and start matching against truly random opponents, especially in the play queues. Given the huge advantage that going first has, there needs to be something else given to the draw player, like maybe a free mulligan or something. You can tell what kind of opponents you get based on the deck you build. Why, when it shouldn't look at your deck and only on your rank/MMR. I'd liked to know how much mana I and the opponent have available to use for all sources, like the number of cards in hand. When having to discard, have a toggle to keep the 7 you want instead of discarding whatever as with big hands, it gets out of control. When searching ours/opponent's deck, organize it so we can look at it faster. Give an option to skip 0 cost, 0 power attackers, etc and effects that wouldn't do anything then (Valki God of Lie's activated ability, Shadowspear when there's no affected targets, etc) from holding priority. Maybe you can have it stop only when changing to battle and end phase or have a toggle that suppresses it until manually activated. If you're not going to ban cards that "infinite combo", limit the amount of actions per turn. Notice the "roping" players and start shortening their time and start penalizing them when they do it a lot. Likewise, when players are going fast in some places then taking their time, give that "banked" time before counting down. In direct challenges, maybe you can agree to untimed rounds or have a thing when the other player is about to start their countdown to extend it. I know it might be due to the way things are resolved but when multiple effects trigger, allow both players the chance to say that they won't respond during this "chain" and then that will allow the game to resolve all effects together and more efficiently. For example, 5 creatures with lifelink attack and deal their damage with Heliod, Sun Crowned on the field. Both players agree to resolve the chain. The attacking player gains 5 life and then will put 5 counters on the proper targets. Once that is done, play resumes. This could be a way to bypass big loops when there's a lot on the stack and when the players agree to resolve the chain, the system will stack like effects and players can only resolve effects and can't activate new ones until the chain is done. This is how we would handle it with paper Magic and as long as both players agree or neither player can activate effects in the chain anyways, let it happen. Nobody enjoys 50 Scute Scarms coming in with Soul Warden(s) and Heliod and no player can activate anything anyways. Just either remember what the max counters those new tokens could have or just hand wave it and say they can put the counters on any of them/none of them because in the grand scheme of things, I don't think it'll matter too much. Huge quality of life change that will prevent draws or the game crashing, even if it means a bit of deviation from the standard rules.
Mastery/Rewards - I like the Mastery system but for the daily and weekly rewards, why only on wins? Reward us for just playing and do a separate weekly total thing for wins. The daily quests are okay but all of these could be more generous in rewards. More gold, maybe some wildcards too.
Economy - Stuff is expensive. The daily deals may be decent for the gem side but why not on the gold side more often? Something really cheap for gold (featured card perhaps) and a heavily discounted pack as your de-facto login bonus. The biggest limiting factor for deck making is Rare wildcards. Can you have the little wildcard card reward switch between Uncommon and Rare like the other one does for Rare and Mythic and possibly reduce it to maybe 3 or 4 to fill up? Unless you give a way for breaking a higher wildcard into smaller ones or combining smaller ones into higher ones, then this is the most consistent way to get the Rare ones. Show the Vault progress after the first time you claim it too.
Historic - Because this format is digital-only, you should go one of two directions with balancing around the fact that rarity doesn't matter, unlike with paper Magic, as you should assume that everyone has 4 of each card. The first is to errata cards. Even if it doesn't match the card as written, so what? This is your opportunity to balance the card and see how it making the proper changes looks and then could print corrected or similar cards in the future. Take Oko, Thief of Crowns for instance. Increase his cost by 1, decrease the starting loyalty by 1, and change the middle ability to a -1 instead of a +1. This keeps his flavor and puts him in line with other planeswalker's abilities and you have to make a choice about whether to go the food route then ultimate or use the minus and take longer and allow for more counterplay. While this could help with banned cards, it could also help with powerful staple cards like Ugin, the Spirit Dragon. I consider him to be a win condition on his own. Change his cost to 4 generic and 4 colorless so it can't be played by everyone easily, reduce the starting loyalty to 5, change his -X to be: Exile each non-land permanent with mana value X or less, and change the -10 to -8. This hits his own colorless permanents as well making it more risky. You can look at all of the statistics of cards in Historic and hit the ones that used most often that could be brought down some so others can shine. Rogues, counterspells, aggro, Ultimatums, and unfun quick win decks (like Tibalt's Trickery). The other direction is to start banning with a more heavy hand. Shake up the meta and allow new cards and strategies to emerge. I'd ban single card win conditions that once played, pretty much means the end with little counterplay (Tibalt's Trickery) or alter the board state so much, that the opponent can't continue (like Ugin or Ulamog, the Ceaseless Hunger) and can be played before they could normally be. Some counterspells that are too versatile or efficient. I'd say any 2 mana value ones are good targets with no way the opponent can pay out of it (Negate, Essence Scatter, etc) especially ones like Drown in the Loch or Dovin's Veto, where it's does everything and the downside isn't a thing with the deck it's used in. Because counterspells are in the same color as drawing cards, the only way to get around them is to have your own counterspells or play aggro decks which makes a lot of cards useless in the meta unless you start making more uncounterable creatures/big bombs. Looking at the aggro or other mana-efficient decks and their mana-efficient cards and hitting the biggest offender of each to slow them down: Thieves' Guild Enforcer, Into the Story, Selfless Savior, Kor Spiritdancer, Heliod, Ruin Crab, Brazen Borrower, Cauldron Familiar, Thoughtseize, Fervent Champion, Skirk Prospector, Robber of the Rich, Bonecrusher Giant, Krenko Mob Boss, Muxus Goblin Grandee, Embercleave, Edgewall Innkeeper, 2 MV Elf Lords, Lotus Cobra, Lovestruck Beast, Marwyn the Nurturer, Dreamtrawler, Yorion Sky Nomad, Lurrus of the Dream-Den, Risen Reef, Zenith Flare, Kroxa Titan of Death's Hunger, Journey to Eternity, Lucky Clover, and Lotus Field. This hits a lot of different decks and while it hurts them, they can still do their main thing, just not as efficiently with the exception of cycling decks but now they can focus on the other cards instead of racing to find the Flares. The last ones should be too easy combo pieces or infinite combos where once it happens, it goes until you win or the opponent loses that turn, either automatically or after taking a lot of actions. Like Exquisite Blood, Thassa's Oracle, Birgi God of Storytelling, Neoform, Valki, Unburial Rites, Scholar of the Lost Trove, Time Warp, and Tibalt's Trickery of course and others I'm forgetting about where once they do it, you scoop because they'll win after taking a ton of actions where you can't interact.
If you do just the Deckbuilding and Gameplay parts, I'd consider that a big win. That would help out a lot and some would be pretty easy to implement. The Rewards/Economy would improve engagement with the game because as right now, most play for the daily quests then that's it if that. The Historic changes would be really nice and I'll doubt it'll happen but it would be nice if Historic was more than just either broken combos or Standard+. I can't believe I took the time to find the cards. I like to see the brew decks and the interesting interactions that people find with the varierty of cards that we have available. Help us get the cards then keep the format in a place where we can have some success no matter what or reward us for playing, regardless of our outcome and we'll enjoy the game that much more.