Maybe get the win in the fastest time possile could be a thing. Only hard part is killing 100 other players...
not sure if we're talking about the (probably abandoned) Fornite Singleplayer game or Fornite Battle Royale here. I'd assume if the singleplayer game was finished (I never heard news about that and don't care about the franchise) that'd be speedrun-able, but Battle Royale likely isn't a good idea.
as ex-dota2 speedrunner i can say for sure - don't waste your time on that game. you won't build a decent community off the popular mmo game, people won't understand the reason why you're doing this and will utilize speedrun strats for malicious things (cleaning up lpq ie)
same applies for every other mmo game, like dead by daylight's (this game is horrible, don't waste your time on it) solo categories, where you basically have to kill 4 players running across the map in the random places and directions. unless you want some memes on your history.
tl;dr
lazy to edit update:
even if you're super good at the game you're running, you have to consider the fact that people that play the game casually (let's say the majority of dota2 players are ~2.7-3k) will have zero clue whats happening, unless someone will explain them what will happen. in general, dota speedruns are interesting to route, but because of the patches theres a lot of things that you can't really plan and you can't really reverse the game's version OR access the maps/events in a legit way (w/o using the workshop). the moment you find the optimal strat, and grind the fuck out of the patch, there's nothing left around. glitches are nonexistent, bugs(huge ones) are getting fixed in the ~60 min. after patch gets released, while the people are literally playing against the time to abuse the fuck out of these bugs (siltbreaker event, dark moon techies exploit). i have a bunch of conversations w/ dota panel guys/pros, and the only "positive" answer was:"well, speedrunning events like dark moon is kinda interesting to do on stream, but i can't really see the future of dota speedruns, and you wrote the exact reasons why, so i don't think i should go any further than that", i had the interview posted on kotaku, i even had a bunch of r/dota2 frontpages, but if you'll take a lazy look at current dota2 leaderboards, you'll see that they are pretty empty, and i don't want to even attempt to waste any more time to re-revive the community. i failed myself as a mod of dota2, and i will never forget myself for that.
Fortnite single player is getting updated at the same pace then the br, even if the playerbase is probably dead, the game isnt. but yeah speedrunniong early acess game/ever changing games/pvp game isnt a good idea