Try adding a health bar or icons that indicate your characters sanity or the health bar indicating their health. The sanity helps the runners see less of the monsters, but lower sanity increase the rate obstacles appear on screen. The health indicates how much health the runner has and may change their running speed. Add some moving enemies so that game is a bit more challenging, for example: the weird head thing is behind a walking tall creature, slide under him then it leads to the floating head thing.
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