I must admit that Fortnite - or Battle Royale shooters in general - are not my scene, but I've always been able to appreciate a good replay editor. It's one thing to play back a match for highlights, but another thing entirely to be able to cut, pan and frame shot after shot for maximum drama, and be able to analyse exactly what your opponents did so wrong in order to get killed by you.
As part of a general suite of upgrades coming to the Unreal 4 engine, Fortnite will be used as a showcase for the new replay editor tech, before the tools become available for all Unreal developers to use later in the year. Within, a shiny new trailer cut with the tools, along with its Making Of dissection.
Well, that was a nicely produced trailer. You'd think it might be an official production (it does look very much like Epic's own looks at the game, albeit a little more bombastic), but apparently it's the work of YouTuber Ali-A, who seems... loud, and enthusiastic in ways that I have never been in my life. You can see how he assembled the trailer in this accompanying video.
The replay editor isn't exclusive to mouse/keyboard play, or even PC, and apparently the interface is designed to be smooth and accessible enough for gamepad use. I can actually see this being optimal in some cases, especially if you want a smooth, measured pan or camera arc - there's just some things that analogue sticks can do better than a mouse's inherent twitchiness.
Epic haven't quite pinned down a launch date for the replay editor system, although do promise that it'll be 'soon', followed not long after by the launch of the next Unreal 4 engine iteration.