Adding a level editor to a game can diversify the player base. Richard Bartle enumerates 4 distinct player types, and appealing to a variety of these player types can increase this player base. The types include Socializers, Killers, Achievers, and Explorers. If your game targets Socializers(eg: fable, sim city, Killers(eg: halo, overwatch), or Achievers (eg: puzzle games), and you want to appeal to a broader base to include Explorer types (eg: Minecraft), include a level editor in your game.
Far Cry Instincts Edit
Far Cry Instincts appeals to killer player types as a competitive fps. Playing this game growing up, I always enjoyed the thrill of this competition. My brother, on the other hand, found much more appeal in the level editor of the game. He represents an explorer player type, and without this aspect of the game, the two of us would not have played it together as much as we did.
Counter Agents Edit
Counter Agents, a game made by Indie developers James Montgomery and Jeff Jackman from the Engineering Arts and Entertainment Game Design Masters Cohort 6, targets socializer and achiever player types, and broadens its player base by including a level editor. The game pits players against each other in 2-4 player coop where players must secretly grab a red briefcase and get to an exit before being caught by guards, which appeals to the socializer player type who enjoys the player-player interaction. The game also features a 1-player mode for the achiever player type and a series of levels they must complete as mini puzzles. The game also includes a level editor for those interested in deeper gameplay and more novel exploration of the world, broadening the variety of appealing components.
Free download: http://store.steampowered.com/app/549090/
Doom (2016) Edit
The most recent Doom game features an extensive level editor called SnapMap. Players can create their own levels and gameplay types by using the different tools in the game. Players can place different assets to create the level and then they can change gameplay like enemy AI and player speed. Players can also add particle effects and a bunch of other gameplay items to create their own game inside of Doom.