Where to begin? The vanilla game isn't very hard, you can enable hardcore mode but all this does is punish you for dying, it doesn't make the actual gameplay that much harder. There should be more hostile mobs with unique mechanics (1.21's Breeze looks promising ngl), as I feel this would make the game more challenging, but in a fun way. I also think that, especially with alot of new blocks being added, that the vanilla inventory is too small, literally just one more row of inventory space would be a massive improvement. Some might say there's not enough content in vanilla, but I honestly think that Vanilla Minecraft should be simple, with mods for adding extra content.