Getting hooked on the booze. Or on some specific potion effect.
Tiered of seeing players spam food every few seconds? Over eating makes you slower. Full hunger with full health makes you slow.
Working like a shurker charge.
Gravity changes based on the position of the moon.
At night when the moon is up you can jump higher and glide along the ground. But can only jump up a half step in the day time.
The dark kills you like in the deep dark only in the overworld.
A pedestal like LoZ that you can place an item into and it repairs or recharges it.
Piping in materials to do the repair. X item for Tcon.
Sand blocks that fall up instead of down. Could relate to Snad. And ether have to be placed on a slime block to stick it down, have a second two to place the crop, or grow the crop upside down.
Mobs hidden in stone and ores that work like silverfish blocks. Releasing mobs resembling the block they spawned from but attacking like chain chomps.
A mod that reproduces the tank from Zycraft.
A tank that could be made out of any solid blocks up to very large sizes.