The social and collecting part of Magic as a trading card game is something that Arena severely lacks. I think as it is now it really leaves players without community in the client and lets players, especially new ones, lose interest very fast. Building a collection is the biggest part of Magic for a lot of players, and as it is now Arena doesn't feel good to collect in. So I have a few suggestions I personally would like to see:

"Guilds": Having a guild type system would extremely help a lot. Being able to join groups of people to be able to chat with and having a more direct way of challenging them would make it very easy to foster communities in the game. This would also helps new players a lot since they usually just get throw into the game after the tutorial and told to swim, and a lot of them get overwhelmed. If not that than even having a "rookie help" channel where people who are ambassadors to the game could reach out and help them.

Binders: Such a classic part of every card game, I think having a binder system that you can have on your profile and others would be able to see would be really cool. You would get a classic 9 pocket binder that you could flip through and put all of your favorite cards into and be able to show off all the cards and styles you've collected. And as a way to earn money from it there could be special binder covers you can buy in the shop.

Better set completion tracking/rewards for completing an entire set. I think there should be a special reward for completing a set. Get 1 of every card in a set, maybe you could unlock some styles for cards from that set. Getting 4 of every card from a set should give you something very very special like card sleeves or something.

I am very passionate about this game, and I do think Arena is a very convenient way to enjoy the game. But as it is right now its still so so far behind every other game in its market and I haven't seem them wanting to improve in these areas in anyway really. So i hope this reaches somebody, and I would love to see these added and I really think it would help the game. Thank you