Actual suggestions at the bottom for the TL;DR crowd but this needs more context.

This game is only engaging through fear of missing out... checking in daily to see what deals are on the store and to complete daily objectives that payout pennies worth of game currency and rewards. Everything is with the hope of a change of a better reward. The daily objectives only reward you for completing generic game actions (playing lands or casting spells of particular colors) or killing your opponent's creatures. The latter really doesn't belong on the list. There aren't any blue-specific action like counter X spells so the rewards system is skewed away from certain colors and styles of play. Furthermore, it leads players to opt into strategies that simply aren't fun in a 1-v-1 format... decks that are all answers, but even decks with no win conditions in them are viable on Arena where you can simply crush your opponent's soul to win - by making the game so miserable to continue they quit.

The latter has always been the case in competitive play, but that dovetails into my other problems with this game. It's purely competitive focused. The mainstay of daily objectives... winning 15 games, not just playing 15 games, but winning them... the game engages people to win in every format that's available where they think they can get an advantage. It doesn't matter how many "competitive" formats and queues you create, they're all competitive (and I mean this in the sense of competitive vs casual - yes, I know the game is always "competitive" in the generic sense. What does all this mean? Well, people largely play to dominate their opponent as quickly as possible so they can complete their dailies and get off the game as fast as possible. You're engaging people that don't want to be there to schlep through a miserable experience so they can get their rewards. The engagement at its core isn't to simply login and have fun... it's to efficiently win and spend as little time playing as possible. These same miserable people are watching videos or on social media or anything else to get through the chore of playing... then they rope you either by rage-quitting entirely or just moving their attention away from Arena to the other activities they're participating in simultaneously.

This dovetails into my next topic... the in-game "community" for this game is awful. For all the above reasons, it leads people to be terrible to each other. It would actually be better if there was an in-game chat and people could actually send unforgivable banworthy messages to their opponents... you'd at least get to weed out some of the bad apples. Instead, all the worst trolls are among the player base and there's nothing anyone can do to actually get banned from the game. Sure, you can report people, but there doesn't seem to be any actions taken... the systemic issues with the game that create these problems in the first place never change.

So... to recap... people like me were just playing as a chore to get rewards in the hope of some day using them to build fun decks and enjoy the game but that never comes. When you grind out all those rewards, it raises your MMR and you begin to only get the worst possible matchups no matter what deck you play and this continues when you switch decks. What this means is just because you killed all your opponents creatures and won as many games as quickly as you could each day (the things the game is forcing you to do), when you do actually log in for fun with your janky casual brawl deck, you get stomped by S and A tier competitive decks in a non-competitive queue. There's just no space for a casual yet engaged player to breath on Arena... maybe if you're a new player who's forces to play casual because you simply don't have the card pool yet... but for players who are just trying to grind the card pool to create casual decks... this game is not for us.

Lessons learned from Arena seem to shape the larger state of Magic... I don't think the majority of the community wants to play games that are decided by turn 4, we just want this on Arena so we can complete our objectives and spend our time doing something we enjoy instead.

That said, you can read between the lines and get several suggestions from my "Dead John letter" but I'll spell out the largest issue and therefor singular suggestion... rework the daily objectives to reward engagement without the need to win. yes, I know a simple "play 15 games" system has it's flaws under the current mechanics of the game... you could simply cast two spells and scoop for the game to count... so there needs to be a serious overhaul in the system without players feeling like something was taken away from them by simply eliminating daily rewards entirely which I am not suggesting. The new systems should allow for players to obtain the same amount of rewards (if not more) - the best avenue for this seems to be to add more interesting and unique objectives besides play lands, kill stuff, and cast spells. Objectives that reward playing "jankier" decks that have fringe mechanics; things that might actually engage players by way of enjoyment rather than toggling through one of three top-tier decks each day depending on the colors you are coaxed into playing that day.