Saying this as a current Reddit user so this is less of a "winning me back" list and more of a wishlist. Either way...
-Reverse API decisions, support 3rd party apps
-Make new design less shit
-Decentralizing. Not through blockchains or NFT trinkets but through open sourcing and federating with a network like this one. This was Jack Dorsey's plan for Twitter and this is why he funded the development of Bluesky. The plan was to eventually develop Twitter into a client for Bluesky, but he had to fuck everything up by pushing for Elon to buy it, which is a shame because this would have been the best ending for the platform. Reddit doing the same would be the best ending for it as well, but like I said, this is just a wishlist. Modern Reddit would never support something like this.