How would you make a modern theme park sim or management
game? Blue sky: an endless runner, or maybe a post-apocalyptic strategy game?
A theme park sim where you have to make an underwater theme park for mermaids where you have to chose what ideas will work best underwater and will be the most enjoyable for the mermaids etc, and live with the decisions you've made even if they don't work. Fish will often destroy your work if they pass in hoards etc, or shark will eat your guests so you have to accommodate for all of that.