Games live and die by their modding capabilities. Some games that have no modding support whatsoever die after a year as everyone who wanted to play them, played them and moved on. They may even be great games, like Limbo. They may offer replay value unrelated to modding (any New Game+ games, e.g. Bastion). They may even be a great procedural experience, such as Enter The Gungeon or The Binding of Isaac. Unfortunately, nothing beats user-created content as the best procedural algorithms currently depend on pre-made "rooms" that game put together (Spelunky, Enter The Gungeon, Biding of Isaac). And "you've seen one, you've seen everything" syndrome is all too common with them.
The solution is simple: Allow create content for your game. Even procedural games based on pre-made rooms can benefit from user-submitted rooms as they'll keep the game fresh on each new run. Let's be honest, internally you have only few designers (few dozens if you're AAA studio) for a given game. By giving your players tools needed to create their own content, you get access to [i]hundreds[/i] potential designers, if not more.
But... your game is a fantasy epic with an actual story? It may benefit from modding as well! Heck, people may buy your game just for the mods. Basically, any game may benefit from it and adding mod support is one of the easiest ways to ensure "long tail" for your game, as moddable games are played even decades after they were released. You can only play "default" levels only so many times even if they are great.
And the best part is that mod tools don't have to be easy and "child-proof", a thing iD seems to be forgetting. Modders will figure out even the weirdest modding tools available. Radiant isn't very user-friendly tool. You have to worry about map leaks all the time. But it didn't stop many Quake players from making their own maps in it. Jazz Creation Station (modding tool for the greatest PC platformer series EVER and if you disagree, you're probably a turtle ;)) isn't easy either, even though it's 2D. Thousands of custom levels exist.
Me? For my own game, Computer Virus Simulator (devlog), I've came up with a fairly sophisticated, yet elegant level editor. I'm coming from an understanding, that if I can make a certain level, players should be able to do so as well.
Computer Virus Simulator's level editor. Sophisticated, yet elegant.
The whole toolset is easy to comprehend once you get the basic concepts right. Since the game uses untextured, flatshaded polygons, you build your levels from the basic 3d primitives. The logic is handled by extensive trigger system, not unlike Quake's or Doom's.
If you after all of this don't think your game is a good fit for modding, ask yourself one simple question: Do I want my game to be played 20 years in the future, or do I want people to forget about its existence after a year or so?