To start, I love the game. It represents Magic well and is fun. But there are some issues with it, which I will list.
I’m starting with things that are common and already well-known.
Put actual text, not placeholder text
By this I mean things like “Leaf_Gilder_Ability” and such. In the case of Leaf Gilder it’s also frustrating not to be able to tell.
Monthly rewards system
Maybe make it a 28 day cycle.
Now there are some things that I’ve seen mentioned less.
Fix the art
Sometimes cards will display the wrong art or none at all, which is minor but annoying.
Make the leaderboards display correct rank
When I sign in, the leaderboards say I’m one higher than I am, and after I play a game it says my rank is zero.
And, finally, some suggestions for making gameplay better.
Get rid of opening dialogue
Between Sorin and Nissa. It’s cringey, unnecessary, and sets a false impression of the game. There’s simply no reason for it.
Make loyalty ability upgrades strictly better
For example, Lv. 3 of Chandra’s second ability (deal three damage to target creature which at Lv. 3 is deal three damage to each creature your opponent controls) goes up in cost (six to nine, I believe) because of an upgrade that typically won’t be worth the cost increase. Why should I level up when it will make one of my best abilities worthless? Don’t make the cost go up, even if you need to make the ability weaker, or maybe allow people to delevel abilities.
Make duplicates useful
Let us trade them for 
or 
. I have two Infinite Obliteration (a mythic). I shouldn’t be disappointed to open a mythic.
Reset the leaderboards and tell us what ribbons are
This and the last one seem like they’re probably in the works, but I’d still like to mention them.
Thanks!
EDIT - Added all commented stuff -
Fix Disabled
As far as I can tell, “Bounding Krasis” disabled is different than “summoning sickness” disabled, but they look the same. Bonded Construct is also shown as disabled, but it just can’t attack, which is different from disabled for other reasons.
Allow User Trading
Yes, I understand that this is kind of unreasonable and likely difficult. It would also be really cool. If I have an extra Infinite Obliteration but don’t play black, I should be able to trade for an Exquisite Firecraft for my red deck. Just an idea.
Make Objectives Easier
For example, the last battle requires you to take more damage than Chandra’s max hp. I’d like to also note red has next to no lifegain and it’s a pain to level up another Planeswalker to a high enough level, especially after the story is over. Also, some require you to destroy a lot of creatures, which blue is bad at, or lose creatures, which can be hard intentionally. And then there’s the “Defenseless” objective which (besides having a broken description) is a little too easy- my red deck has no creatures with defender, given how it’s mostly a downside.
Fix Chandra’s Color Bonuses
This is very minor, but for all Planeswalkers except Chandra, they get (fully leveled up) +0 for enemy color matches (looking at the back of a Magic card, the colors opposite it- for red, white and blue) +1 for ally color matches (the colors next to it- for red, green and black) and +2 for same color matches. Chandra gets +0 for black and +1 for white, which is the wrong way around. Again, very minor, but I’d like to see it fixed.
Make Deckbuilding Easier
Clunky UI. Could be better.
Allow Players to Switch Creature Order
It’s annoying for me to cast a creature, then have it bump another creature off of a much-needed support and kill it (for example, I have a creature that would be a 2/0 and dead but it’s getting buffed by Veteran’s Sidearm to be a 3/1). It’s also annoying when my creatures attack into defenders in the wrong order. At the very least, make creature order be most recently cast last or first (one or the other) and not whatever is going on now.
Make Reminder Text Stay on Screen
I only know what Regenerate does because people here told me. It goes off screen when I try looking.
Allow Support Destruction to Target
It’s incredibly frustrating for a Demolish to hit a Caves of Koilos instead of a Demonic Pact.
Allow Looking at the Board Before Casting Spells
By this I mean when the prompt comes up I should be allowed to see the board before deciding whether to cast the spell. Why? Spell Mastery. Many a Fiery Impulse has gone awry because I thought it would deal enough damage and decided to cast when I shouldn’t have, or didn’t cast when it would have done enough.
Allow Scrolling on Leaderboards
Simple enough. We don’t need to see the whole leaderboard, but being able to see who is in the gap between what we can see and top 10 would be nice.
Make Support Gems Easier to Read
They don’t pop as a gem of their color. I usually see the computer suggest a match, then spend ten seconds trying to figure out what it’s seeing before I notice the support gem. Also, 
support gems look more like 
. It would also be nice if there was something in the tutorial pointing out the little arrows which tell you which gem belongs to which player. I didn’t notice until someone on here pointed them out.
Make Small Creatures Better
In MTG, small creatures are good because you can chump block and life totals are lower. In this game, the lowest max life total is 70 and few creatures can block, which means creatures such as Goblin Glory Chaser, Subterranean Scout, and Caustic Caterpillar are useless.
Have Support Cards Display Durability
By this I mean the number of matches until it breaks. Just put a number in the text box, like “durability 3” for Sigil of Valor.
Show Creature Types
There is at least one creature (Shaman of the Pack) which cares about creature types but no creature types are shown on cards.
Nerf Some Cards
Harbinger, for example, has an OP effect on a body way more efficient than blue should get. Outland Colossus, due to the fact that removal is bad and most creatures can’t block, is difficult to counter. Conversely,
Make Some Cards Better
Mage-Ring Responder is horrible, especially for a rare. And green’s ramp is also horrible. It should have a few cards that outright give more mana than you paid. Same for black’s removal.
Indicate That This Forum Exists In-Game
This forum is an excellent learning tool for a new player. Why not tell them about it?
Add Clans
Probably already in the works, but if not, consider it.
Mention That Only Spells Count As Spells
I know this one sounds weird, but basically, mention in the tutorial or somewhere that only cards marked spells are spells for the purposes of Managorger Hydra and achievements. Coming from real MTG, where they are all spells, I wish I had known this earlier. Very minor.
NEWEST -
Implement EOT Discard
Seeing how bounce now won’t work with a full hand makes me think that there should just be discard down to five at end of turn. This would allow for cards to cycle as well if you have a full but bad hand.
Elaborating on Fixing Daily Rewards
So it’s the 31st as I write this, but today I’m getting my second to last prize. I have a bad feeling I’m not getting my Fat Pack, which is odd because I don’t think I missed a day. Anyway, I think the daily rewards cycle should be 1-2 days shorter than the month itself, or entirely separated from the month. I understand that the entire point is to encourage people to play, but does it really encourage me to check in when I know there’s no way I’ll get the ultimate prize just because I missed a day? Maybe another option is give players 1 free day a month. To get the reward, they still have to check in daily, but if, say, they’re travelling, tired, busy, or just forget, it doesn’t make the Fat Pack impossible to get for that month and reset them back to lower-level rune prizes.
FIX PRICES
This one, I feel, is very important.
Obviously, you can’t make everything free. And I love this game. But it’s hard to justify buying crystals when the prices are so absurd. For example, in one of the best deals I could get 1,300 crystals for 49.99. That’s 29 packs (two Big Boxes and one normal pack), which is 145 cards. Or I could spend that 49.99 on a fat pack (~30.00, though MSRP is 39.99) of real MTG cards and a Deckbuilder’s Toolkit (19.99). Fat packs have nine packs of 15 cards and 80 basic lands (which are required to play)- that’s 215 cards, which have real value, can be traded, and are not useless in multiples. So I have already done better than this. And a Deckbuilder’s Toolkit contains 285 cards and is great for new players.
So to recap- I can spend 50 dollars on 145 valueless virtual cards, many of which will be useless duplicates and which I can only use to play against bots, or 50 dollars on 495 real cards, some of which will have real value and which I can use with other people, and have a good start to a MTG collection along with some nice storage boxes.
I know which one I would choose and which one I have chosen.