That's what Epic has been doing though. Rocket Racing and Lego Fortnite were technically inside the Fortnite client but are their own games. Not like mods or modes, but actual games. However, none of those games got to the level of engagement of Fortnite Battle Royale, so like Valve after Artifact or Dota Underlords, or like Epic itself after Paragon, it has to try and experiment further until there is a hit that justifies the investments and gives them more.