According to Fortnite: Battle Royale Leaks Twitter and several other sites, it seems that the game will be getting Creative Mode starting in Season 7. This means that players will have access to tools to create unique private islands as well as have the ability to host custom servers. Players can host limited time servers or even leave them up for others to join. Server selection will take place via a central lobby.
From what has been shown by YouTuber Lachlan, players will have tons of options for weapons, building materials, terrain, props, mini-games and more. Players will also be able to choose rulesets for their servers and much more.
Fortnite Intel has a nice write up about the Lachlan video that includes a number of screenshots and commentary from the video that show off server selection, spawn points, server setting selections, and much more.
Learn more about the upcoming mode at the link provided above and in the video.