Some things to chew on:
Top right:
- What's the theme of these eight puzzles?
- What's behind the door?
- Heck, what's behind the door in front of you?
- What's underneath the boulder?
Bottom right:
- What is it called when someone or something is expected to have arrived already?
- Who might be interested in such footprints, and what might it represent to them?
And as always, a good starting point is using the letters/sounds from one phrase in the other.