This would make more sense as a datapack or map, but I don't have the skills for that, so I just made a mod in MCreator.

Blocks and mobs fall from the sky in this challenge mod. At night, those spawn much faster, and this can be tricky to survive with the many mobs and explosives ready at all times for the block to spawn.

Instructions:

Make a void world (to do this, make a superflat world and change the preset to void) and start in creative mode.
Change the material of the stone platform or keep it the same. Flammable blocks add extra challenge.
In the creative inventory, scroll all the way down in the "Functional Blocks" tab or search "Challenge Core."
Place the block at the center of the platform.
Go to survival mode.

Tips for Starting:

Make a house with a roof as soon as possible. Night has faster block and mob spawns, so it is even more dangerous.
Make it out of strong materials; explosives like TNT, creepers, and fireballs can spawn as well as fire and lightning.
A blaze is the toughest mob that can spawn.

Falling Blocks Challenge