After you have beaten the final boss in the game, it seems there is not many options of what to do.
In the comments list things you have found are enjoyable once beaten the game.
1. Buy the boss' armor from The Beggar in Castle of Storms for 200k gold (or 150k if you don't like wearing helmets like me). 2. Upgrade that shit to the max (requires drowned-series upgrade items). 3. Be a boss in your next playtrough in a quest to make every sanctuary on the island into a Betrayer creed sanctuary. 4. Upon beating The Nameless God that this is eminently purposeless as you are trapped in a limbo. 5. Go join the goldfaced, wineswallowing drunks. 6. Spread joy and prosperity to the island in your next playthrough.