So we could have inkwell isle 1 2 and 3, with different rules such as run n guns, grades and difficulties. For example: (RnGs is run n' guns)

Inkwell Isle I All Bosses Simple Inkwell Isle I All Bosses Regular Inkwell Isle I All Bosses Expert Inkwell Isle I All Bosses Simple + RnGs Inkwell Isle I All Bosses Regular + RnGs Inkwell Isle I All Bosses Expert + RnGs Inkwell Isle I Grades S+P

Repeat for II and III.

All categories should end by completing the game. What you suggested is just NG+ but for only one isle.