Minecraft wiki entry for Hardcore_mode, associated text: Do massive projects, for example, covering your end in ice or making an overworld in your nether or end island.
Make farms like a creeper farm, experience farm, villager breeding center, iron farm, etc.
Make a full set of netherite armor and tools with the best enchantments.
Make a fully powered beacon out of diamond blocks (if not too lucky in general at least make a fully powered beacon out of iron blocks.)
Fight the wither.
Explore natural generated structures like the desert temple or the ocean monument.
For an extra challenge, return to the End. Use four end crystals to re-summon the ender dragon.
If you thought Hardcore was too easy, there are extreme survival maps such as Skyblock and mods such as Skyzone (a Skyblock generator) and Ultra Hardcore.
Invade a woodland mansion.
Conquer a Bastion Remnant.
Take on a Raid
Try finding all the biomes found in Minecraft. There are so many to be found!
Complete all of the advancements.
Play on the Single Biome world type, and maybe challenge yourself with a biome that is harder to survive in, such as Ice Spikes, or Mushroom Fields.
Play on a default Superflat world , and make as much progress as you can.
Challenge yourself by doing a Nether Survival challenge. (You start in The Nether, progress through the game without leaving The Nether, eventually kill the Wither, and finally, build a portal back to The Overworld.)
Made all types of beacons in your world.
Tame all the parrots colors
Tame 5 wolves