It was inevitable, wasn't it? multiplayer last year, now has a battle royale mod called Battle Royalley. If enough people start using it, you should be able to 100-player fights to the death, complete with loot chests housing swords, bombs, slingshots and healing items. Last farmer standing wins.
Players will be spread around the map at the start of each round and the play zone will gradually shrink until, eventually, the battle is confined to a small corner of the map. You can chop trees and craft fences to use as cover, hide behind bushes for surprise attacks, and jump on a horse to gallop away from a sticky spot.
If you host a server, you can choose where the storm closes in on, as well as pick chest placement and player spawn locations. Instructions for how to do that, along with an installation file for the mod, can be found on its Nexus page.
If you host, you can also choose exactly how many players you want to a match. The mod's author Ilyaki suggests 100—I'm not sure how well it will run (Stardew Valley defaults to four-player multiplayer, although you can increase that with mods), and presumably it'll be rare to get that many people playing the mod at once. But this is the kind of thing that could take off, at which point you'll probably be able to find full games.
Here's a short clip of what to expect.
If you have lives here.
