The Nintendo UK site for Super Mario 3D World + Bowser's Fury has shared a few more details on what the Bowser's Fury mode has to offer. You can read a summary of that info below.
- Bowser has been tainted by a mysterious black goop, which turns him into Fury Bowser
- the shell that we see Bowser trapped in is called the Fury Sun
- the Fury Sun will always rise up near the island you're on
- weather will change, which indicates how much time you have until the Fury Sun emerges
- if you’ve collected five Cat Shines on an island, you can activate the Giga Bell
- this is what transforms Mario into Giga Cat Mario
- defeat Fury Bowser and he'll be sent flying back into the water, which unlocks new islands for you to explore