On Sandfall island (or somewhere else idk) there is a Grand Mirage, which you can interact with to start the Mirage Tournament. This is a boss rush that spans every boss in the Bronze Sea. Earlier bosses will have health and damage scaled up.
seems a little much for just beating scaled up bosses
none of them were hard on the first fight besides calvus and the only difference now is that you have better gear, more skill and more abilities so it seems pretty effortless
Idk about the rewards but a boss rush definitely would be fun, and its also and endgame content so players wont get bored as quickly after finishing the story of a sea when the next sea isnt out, or if it is out this would be just a fun addition. Would like to see something like that for each sea tbh.