I have ideas already, here are some:
Beat game as a cat
Beat game as a wolf
Beat game as a fox
Beat game as a skeleton horse (this one would probably be with a friend)
Survive in an all water world (1.12.2)
Survive 9b9t
Kill the Wither in a Floating Islands version of the end
Beat the game as an axolotl
Axolotl Manhunt (with friends)
Kill the Killer Bunny
Completing the list of Husbandry advancements
Survive on the GTA server
I need more ideas, give me some.