My son comes up with novel themes all the time while he designs playfields. Some of my favorites were:

Ice Cream Shop (he thought of this theme as a pun on "scoop" and went with it, had cool modes and multiballs collecting flavors, filling orders, getting raw ingredients and such)

Solar Madness (the outer planets all had modes and rules, there was an "orbit" around Saturn for the rings, the pops had hidden standups and were the asteroid belt, Pluto was a behind the field ramp shot for a lock, the inner planets were an under the table playfield ala' Black Hole, etc... pretty cool)

Backpacker (pretty easy to imagine this, equipment to collect, mountains to climb, hazards to avoid, there was a random reward / mode start saucer where he said the backglass would have an animation where the guy complained about prices)

I have more than 100 of these things.

I'd put my eight year old's designs up against some random 10 years old any hour of any day of any week.