I want to maintain exactly the same game balance. To introduce cool things that don't change balance:
Make visual pieces such as atmospheric items
Make characters
Make interesting stories
Make different things, not more. Rather than making more powerful items, make different items. Make items that are better for certain situations or more beautiful.
Maybe an item that is particularly powerful on one day out of the month.
Make encounters challenging by making it difficult to escape. Make a door lock behind the player and such.
I want to maintain exactly the same game balance. To introduce cool things that don't change balance: