A ton, actually. I’m not very engaged in it, but every time some friends convince me to play something on Fortnite with them, I can’t help but notice the section of hundreds and thousands of gamemodes with custom rules, items, mechanics, and so on.
For example, I see many clones of bed wars from Minecraft, third-person CoD clones, RP servers, base/colony building gamemodes, zombie survival modes, etc. It effectively has a sandbox game maker inside of the regular Fortnite, alongside Rocket Racing (racing game) and Fortnite Festival (rhythm game). It’s a very fleshed out environment, although it doesn’t have the same potential for custom assets that Roblox does.