Add a command to pause and a command to unpause Minecraft.

Idea: when entering a world for the first time, pause the game and show a custom fancy menu ui to begin the mod pack story arc, or immediately play a video.

Commands to pause and un pause the game would allow me to add in story screens without worrying the player might die if it's at night. Obviously doesn't work on a server.

I have the player falling from the sky at the beginning of a new world. This would allow me to begin the story before the player falls and hit the ground.

I could put the player into spectate mode, but then all the sounds and music still play and it limits what's possible.

There's no mod I can find that allows me to pause and un pause the game with a command.

Obviously won't work on servers.

Custom GUIs should pause the game by default when they are open.
That only works in SP.