I'd argue it's a lot more than somewhat derivative.
The game mechanics are a direct rip-off of Player Unknown Battlegrounds, except more 'kid friendly.' The only notable differences are being cartoonish and the addition of a building system (which looks a whole lot like the one in Rust). Here is a side-by-side video of gameplay https://www.youtube.com/watch?v=2KN6_Ibxnpk.
Their main source of income is selling in-game emotes like dances, which are all stolen without paying any royalty to their creators.
Probably missing a bunch of steps but you get the picture.
COD Zombies doesn't have building other than at specific points where you could reinforce barricades.
At the time the first COD Zombies game mode was released (Call of Duty: World at War) Battlefield did not yet have a barricading system (like it does now) or any sort of wave-based survival mode.
Quake is a deathmatch shooter, Battlefield is a tactical shooter. Very little resemblance.
Quake was made by the same people as Doom.
My point is that Fortnite copies very much from PUBG. I didn't make any arguments about whether that is acceptable or precedented.
Please read what I said and don't put words in my mouth.
This is just wrong. The game became popular because its a pretty damn good game and quite fun. Yes, it is somewhat derivative but most things are. People forget that Epic is an old school titan of the gaming industry that pioneered a lot of what we see in FPS titles today. They know what they're doing, even if they lifted some inspiration from other titles.