No plummeting hack

When characters are out of the map and in the gigantic blue thing, if they jump they plummet for who knows how long. I'd like a hack which stops that from happening. Even if the only way to do that is stopping the characters from being able to jump in the gigantic blue thing, that could be done.

But I don't know if it's possible to only do that while outside the map, because inside the maps the characters need to be able to fall if they jump from a high place, and the computer probably doesn't know the difference between the inside and outside of the map

The plummeting in question from a technical point of view is impossible to prevent. The game indeed doesn't know the difference between the game map and the out of bounds area. It looks for the collision below the player, and if it can't find any - the game will simply put the character into a falling state indefinitely, until he lands somewhere
There is a way to stop it - press F7 with cheat keys to teleport your character forwards, this will trick the game
Other than that - there is no need nor point in creating such a hack. You're not supposed to go out of bounds during regular gameplay anyways

Oh ok, what's the name of the mod that has it so pressing F7 teleports your character forwards?

cheat keys it comes with lucas mod launcher