If you want to increase the challenge, add in Stage II escalations! Most Stage II cards show a fort symbol. When cards with a fort symbol would be used to Explore, first perform this action:

On each Board: Add 1 Town to a land without a City.

This means that those lands will now be a source of Explorers, making the Explore action on those cards more dangerous. Another way to increase the challenge of the game is by adding an Adversary or Scenario from the original core game of Spirit Island or one of its expansions. These products also contain additional Spirits, Power Cards, Fear Cards, Events, and even more gameplay options!