How "easy" or "hard" is making a custom story?

[ZyLogicX]

Lets say it in a nutshell

Everyone can build a game, either if its the Source Engine,Unreal engine,CryEngine or HPL engine.

It depends on wheter you can create innovative gameplay, and not copy parts of the game. (You could, but noone is interested in that.)

Scripting is necessary for a good custom_story, You don't have to for certain things (like scripting pathnodes for the monster) but it usually is better.